mpv: set meta.mainProgram

pkgs/applications/video/mpv/wrapper.nix

···
         rm "$out/Applications/mpv.app/Contents/MacOS/mpv"
         makeWrapper "${mpv}/Applications/mpv.app/Contents/MacOS/mpv" "$out/Applications/mpv.app/Contents/MacOS/mpv" ${mostMakeWrapperArgs}
       '';
+
+      meta.mainProgram = "mpv";
     };
 in
   lib.makeOverridable wrapper