endgame-singularity: add option to disable the default music pack

It's the heaviest part of the game. People may not want to have it take
space or may want to use their own music.

pkgs/games/endgame-singularity/default.nix

@@ -3,6 +3,7 @@
 , fetchFromGitHub
 , unzip
 , python3
+, enableDefaultMusicPack ? true
 }:
 
 python3.pkgs.buildPythonApplication rec {
@@ -16,18 +17,19 @@ python3.pkgs.buildPythonApplication rec {
       rev = "v${version}";
       sha256 = "0ndrnxwii8lag6vrjpwpf5n36hhv223bb46d431l9gsigbizv0hl";
     })
-    (fetchurl {
+  ] ++ stdenv.lib.optional enableDefaultMusicPack (
+    fetchurl {
       url = "http://www.emhsoft.com/singularity/endgame-singularity-music-007.zip";
       sha256 = "0vf2qaf66jh56728pq1zbnw50yckjz6pf6c6qw6dl7vk60kkqnpb";
-    })
-  ];
+    }
+  );
   sourceRoot = "source";
 
   nativeBuildInputs = [ unzip ]; # The music is zipped
   propagatedBuildInputs = with python3.pkgs; [ pygame numpy polib ];
 
   # Add the music
-  postInstall = ''
+  postInstall = stdenv.lib.optionalString enableDefaultMusicPack ''
     cp -R "../endgame-singularity-music-007" \
           "$(echo $out/lib/python*/site-packages/singularity)/music"
           # ↑ we cannot glob on [...]/music, it doesn't exist yet