mpv: Set `meta.mainProgram`

pkgs/applications/video/mpv/default.nix

···
     '';
     changelog = "https://github.com/mpv-player/mpv/releases/tag/v${finalAttrs.version}";
     license = licenses.gpl2Plus;
+    mainProgram = "mpv";
     maintainers = with maintainers; [ AndersonTorres fpletz globin ma27 tadeokondrak ];
     platforms = platforms.unix;
   };