assaultcube licensing: mark unfree

The license of AssaultCube as a whole is not zlib, it's freeware, see: https://assault.cubers.net/docs/license.html

closes #121381

pkgs/games/assaultcube/default.nix

···
     homepage = "https://assault.cubers.net";
     maintainers = [ ];
     platforms = platforms.linux; # should work on darwin with a little effort.
-    license = lib.licenses.zlib;
+    license = lib.licenses.unfree;
   };
 }