endgame-singularity: add "mainProgram"

Withtout the change `nix run` fails to run the game:

$ nix run -f. endgame-singularity
error: unable to execute '...-endgame-singularity-1.00/bin/endgame-singularity': No such file or directory

pkgs/games/endgame-singularity/default.nix

···
       free # earth images from NASA, some fonts
       cc0 # cick0.wav
     ];
+    mainProgram = "singularity";
     maintainers = with lib.maintainers; [ fgaz ];
   };
 }