gnu: solaar: Fix build.

* gnu/packages/admin.scm (solaar)[arguments]: Remove unnecessary custom ‘build’ phase.
Set PYTHONPATH earlier, before building.

Reported by Robin Templeton (terpri on #guix).

gnu/packages/admin.scm

@@ -3673,10 +3673,7 @@ (define-public solaar
               (("'--prefix' in sys\\.argv")
                "len([x.startswith('--prefix=') for x in sys.argv]) > 0"))
              #t))
-         (replace 'build
-           (lambda _
-             (invoke "python" "setup.py" "build")))
-         (add-before 'check 'setenv-PATH
+         (add-before 'build 'setenv-PATH
            (lambda _
              (setenv "PYTHONPATH" (string-append "lib:" (getenv "PYTHONPATH")))
              #t)))))