i3lock-color: add mainProgram

pkgs/applications/window-managers/i3/lock-color.nix

···
     '';
     homepage = "https://github.com/PandorasFox/i3lock-color";
     maintainers = with maintainers; [ malyn ];
+    mainProgram = "i3lock-color";
     license = licenses.bsd3;
 
     platforms = platforms.all;