xonotic: fix desktop item

The desktop item was referring to "$out/bin/xonotic" as executable, but
since it's built in its own derivation now, "$out" did not contain the
xonotic wrapper.
Since it seems to be common practise to use a relative path, I changed
it to do that here as well.

pkgs/games/xonotic/default.nix

@@ -45,7 +45,7 @@ let
 
   desktopItem = makeDesktopItem {
     name = "xonotic";
-    exec = "$out/bin/xonotic";
+    exec = "xonotic";
     comment = meta.description;
     desktopName = "Xonotic";
     categories = "Game;Shooter;";