gnu: ffmpeg: Use 'modify-phases'.

* gnu/packages/video.scm (ffmpeg)[arguments]: Use 'modify-phases'
  instead of 'alist-cons-after' et al.

gnu/packages/video.scm

@@ -323,7 +323,8 @@ (define-public ffmpeg
        #:imported-modules (,@%gnu-build-system-modules
                            (guix build rpath))
        #:phases
-         (alist-replace
+       (modify-phases %standard-phases
+         (replace
           'configure
           ;; configure does not work followed by "SHELL=..." and
           ;; "CONFIG_SHELL=..."; set environment variables instead
@@ -409,8 +410,8 @@ (define-public ffmpeg
                       "--disable-mips32r2"
                       "--disable-mipsdspr1"
                       "--disable-mipsdspr2"
-                      "--disable-mipsfpu"))))
-       (alist-cons-after
+                      "--disable-mipsfpu")))))
+         (add-after
           'strip 'add-lib-to-runpath
           (lambda* (#:key outputs #:allow-other-keys)
             (let* ((out (assoc-ref outputs "out"))
@@ -419,8 +420,7 @@ (define-public ffmpeg
               (with-directory-excursion out
                 (for-each (cut augment-rpath <> lib)
                           (append (find-files "bin" ".*")
-                                (find-files "lib" "\\.so\\..*\\."))))))
-          %standard-phases))))
+                                  (find-files "lib" "\\.so\\..*\\."))))))))))
     (home-page "http://www.ffmpeg.org/")
     (synopsis "Audio and video framework")
     (description "FFmpeg is a complete, cross-platform solution to record,