gnu: Simplify "Xvbf" invocation in pre-check phases.

* gnu/packages/geo.scm (qgis)[arguments]: Simplify invocation of 'Xvfb'
in 'check' phase, assuming 'Xvfb' is in $PATH.
* gnu/packages/graphics.scm (opensubdiv)[arguments]: Likewise.
* gnu/packages/java-graphics.scm (java-piccolo2d-extras)[arguments]:
Likewise.
* gnu/packages/kde-frameworks.scm (kcontacts)[arguments]: Likewise.
(kdeclarative)[arguments]: Likewise.
* gnu/packages/kde-multimedia.scm (elisa)[arguments]: Likewise.
* gnu/packages/kde-plasma.scm (kscreenlocker)[arguments]: Likewise.
* gnu/packages/radio.scm (gnuradio)[arguments]: Likewise.
* gnu/packages/education.scm (gcompris-qt)[arguments]: Likewise.
* gnu/packages/freedesktop.scm (weston)[arguments]: Likewise.

gnu/packages/education.scm

@@ -157,8 +157,7 @@ (define-public gcompris-qt
          (add-before 'check 'start-xorg-server
            (lambda* (#:key inputs #:allow-other-keys)
              ;; The test suite requires a running X server.
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                    "/bin/Xvfb :1 &"))
+             (system "Xvfb :1 &")
              (setenv "DISPLAY" ":1")
              ;; The test suite wants to write to /homeless-shelter
              (setenv "HOME" (getcwd))

gnu/packages/freedesktop.scm

@@ -1134,8 +1134,7 @@ (define-public weston
          (add-before 'check 'start-xorg-server
            (lambda* (#:key inputs #:allow-other-keys)
              ;; The test suite requires a running X server.
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                    "/bin/Xvfb :1 &"))
+             (system "Xvfb :1 &")
              (setenv "DISPLAY" ":1")
              #t)))))
     (home-page "https://wayland.freedesktop.org")

gnu/packages/geo.scm

@@ -2138,8 +2138,7 @@ (define-public qgis
          (replace 'check
            (lambda* (#:key inputs #:allow-other-keys)
              (setenv "HOME" "/tmp")
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                    "/bin/Xvfb :1 &"))
+             (system "Xvfb :1 &")
              (setenv "DISPLAY" ":1")
              (setenv "TRAVIS" "true")
              (setenv "CTEST_OUTPUT_ON_FAILURE" "1")

gnu/packages/graphics.scm

@@ -1708,8 +1708,7 @@ (define-public opensubdiv
                   (add-before 'check 'start-xorg-server
                     (lambda* (#:key inputs #:allow-other-keys)
                       ;; The test suite requires a running X server.
-                      (system (string-append (assoc-ref inputs "xorg-server")
-                                             "/bin/Xvfb :1 &"))
+                      (system "Xvfb :1 &")
                       (setenv "DISPLAY" ":1")
                       #t)))))
     (native-inputs

gnu/packages/java-graphics.scm

@@ -75,8 +75,7 @@ (define-public java-piccolo2d-extras
          (add-before 'check 'start-xorg-server
            (lambda* (#:key inputs #:allow-other-keys)
              ;; The test suite requires a running X server.
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                    "/bin/Xvfb :1 -screen 0 640x480x24 &"))
+             (system "Xvfb :1 -screen 0 640x480x24 &")
              (setenv "DISPLAY" ":1")
              #t)))))
     (inputs

gnu/packages/kde-frameworks.scm

@@ -1641,8 +1641,7 @@ (define-public kcontacts
              ;; Xvfb doesn't have proper glx support and needs a pixeldepth
              ;; of 24 bit to avoid "libGL error: failed to load driver: swrast"
              ;;                    "Could not initialize GLX"
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                    "/bin/Xvfb :1 -screen 0 640x480x24 &"))
+             (system "Xvfb :1 -screen 0 640x480x24 &")
              (setenv "DISPLAY" ":1")
              #t)))))
     (home-page "https://community.kde.org/Frameworks")
@@ -2386,8 +2385,7 @@ (define-public kdeclarative
            (lambda* (#:key inputs #:allow-other-keys)
              ;; The test suite requires a running X server, setting
              ;; QT_QPA_PLATFORM=offscreen does not suffice.
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                    "/bin/Xvfb :1 -screen 0 640x480x24 &"))
+             (system "Xvfb :1 -screen 0 640x480x24 &")
              (setenv "DISPLAY" ":1")
              #t)))))
     (home-page "https://community.kde.org/Frameworks")

gnu/packages/kde-multimedia.scm

@@ -186,8 +186,7 @@ (define-public elisa
            (lambda* (#:key inputs #:allow-other-keys)
              ;; The test suite requires a running X server, setting
              ;; QT_QPA_PLATFORM=offscreen does not suffice.
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                    "/bin/Xvfb :1 -screen 0 640x480x24 &"))
+             (system "Xvfb :1 -screen 0 640x480x24 &")
              (setenv "DISPLAY" ":1")
              #t))
          (replace 'check

gnu/packages/kde-plasma.scm

@@ -154,8 +154,7 @@ (define-public kscreenlocker
        (modify-phases %standard-phases
          (add-before 'check 'check-setup
            (lambda* (#:key inputs outputs #:allow-other-keys)
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                   "/bin/Xvfb :1 -screen 0 640x480x24 &"))
+             (system "Xvfb :1 -screen 0 640x480x24 &")
              (setenv "DISPLAY" ":1")
              #t))
          (delete 'check)

gnu/packages/radio.scm

@@ -579,8 +579,7 @@ (define-public gnuradio
          (add-before 'check 'set-test-environment
            (lambda* (#:key inputs #:allow-other-keys)
              (setenv "HOME" "/tmp")
-             (system (string-append (assoc-ref inputs "xorg-server")
-                                    "/bin/Xvfb :1 &"))
+             (system "Xvfb :1 &")
              (setenv "DISPLAY" ":1")
              #t))
          (replace 'check