ue4: mark as broken

The source code is no longer available as the repository
https://github.com/EpicGames/UnrealEngine has been deleted.

pkgs/games/ue4/default.nix

@@ -78,5 +78,6 @@ stdenv.mkDerivation rec {
     license = stdenv.lib.licenses.unfree;
     platforms = stdenv.lib.platforms.linux;
     maintainers = [ stdenv.lib.maintainers.puffnfresh ];
+    broken = true;
   };
 }