gollum: fix shebang in bin/gollum

In the v5.3.0 release of gollum, the shebang in `bin/gollum` changed,
breaking the package build:
https://github.com/gollum/gollum/compare/v5.2.3..v5.3.0#diff-0108eafd2bcdf5151e078efec0119e63431569fca19b46660c9b8d9b7cdd6cf5R1

pkgs/development/ruby-modules/gem-config/default.nix

@@ -294,6 +294,14 @@ in
     propagatedBuildInputs = [ gobject-introspection wrapGAppsHook glib ];
   };
 
+  gollum = attrs: {
+    dontBuild = false;
+    postPatch = ''
+      substituteInPlace bin/gollum \
+        --replace "/usr/bin/env -S ruby" "${ruby}/bin/ruby"
+    '';
+  };
+
   grpc = attrs: {
     nativeBuildInputs = [ pkg-config ] ++ lib.optional stdenv.isDarwin libtool;
     buildInputs = [ openssl ];