gnome.gnome-robots: add darwin support

pkgs/desktops/gnome/games/gnome-robots/default.nix

···
     description = "Avoid the robots and make them crash into each other";
     maintainers = teams.gnome.members;
     license = licenses.gpl3Plus;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }