snipes: mark as broken on Darwin

pkgs/games/snipes/default.nix

···
     homepage    = "https://www.vogons.org/viewtopic.php?f=7&t=49073";
     license     = licenses.free; # This reverse-engineered source code is released with the original authors' permission.
     maintainers = with maintainers; [ peterhoeg cybershadow ];
+    broken      = stdenv.isDarwin; # not supported upstream - https://github.com/Davidebyzero/Snipes/issues/8#issuecomment-433720046
   };
 }