feat(nix): make stack optional for Darwin builds

The recommended way to build GGTX is via cabal anyway, and stack
shouldn't get in the way of a successful build.

shell.nix

@@ -3,9 +3,8 @@ let
   myBuildInputs = [
     pkgs.pkgs.docker-compose
     #pkgs.pkgs.haskell-language-server
-    pkgs.pkgs.stack
     pkgs.pkgs.websocat
-  ];
+  ] ++ pkgs.pkgs.lib.optional (!pkgs.pkgs.stdenv.isDarwin) pkgs.pkgs.stack;
 in
 pkgs.pkgs.mkShell {
   name = pkgs.shell.name;