mpv: fix umpv.desktop

mpv ships with a wrapper-script umpv that adds files to the queue of a currently
running mpv instance. This script does not take any options.

The desktop file for it simply substituted mpv with umpv in mpv.desktop,
resuting in an incorrect desktop file with `Exec=umpv --player-operation-mode=pseudo-gui -- %U`.
This resulted in umpv adding bogus files to the queue.

pkgs/applications/video/mpv/default.nix

@@ -200,7 +200,7 @@ in stdenv.mkDerivation (finalAttrs: {
     cp ../TOOLS/mpv_identify.sh $out/bin
     cp ../TOOLS/umpv $out/bin
     cp $out/share/applications/mpv.desktop $out/share/applications/umpv.desktop
-    sed -i '/Icon=/ ! s/mpv/umpv/g' $out/share/applications/umpv.desktop
+    sed -i '/Icon=/ ! s/mpv/umpv/g; s/^Exec=.*/Exec=umpv %U/' $out/share/applications/umpv.desktop
   '' + lib.optionalString stdenv.isDarwin ''
     mkdir -p $out/Applications
     cp -r mpv.app $out/Applications