gnu: python-waf: Use a proper directory structure.

Fixes <https://bugs.gnu.org/25634>.

* gnu/packages/python.scm (python-waf): Replace custom 'install phase to
install "waf" binary into the bin directory. Remove the 'wrap phase.
* gnu/packages/video.scm (mpv)[arguments]: Adjust the 'setup-waf phase
accordingly.

gnu/packages/python.scm

@@ -5692,9 +5692,12 @@ so it might be a tiny bit slower.")
            (lambda _
              (invoke "python" "waf" "--version")))
          (replace 'install
-           (lambda _
-             (copy-file "waf" %output)
-             #t)))))
+           (lambda* (#:key outputs #:allow-other-keys)
+             (let ((out (assoc-ref outputs "out")))
+               (install-file "waf" (string-append out "/bin")))
+             #t))
+         ;; waf breaks when it is wrapped.
+         (delete 'wrap))))
     (home-page "https://waf.io/")
     (synopsis "Python-based build system")
     (description