jekyll: fix 'jekyll new'

Original error:

$ nix-shell -p jekyll --command "jekyll new test"
Running bundle install in /private/tmp/test...
  Bundler: There was an error while trying to write to
  Bundler:
`/nix/store/l67429rhvrmr7c4c1msb7s8zjq4fx7ad-gemfile-and-lockfile/.bundle/config`.
  Bundler: It is likely that you need to grant write permissions for
that path.

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

@@ -78,6 +78,15 @@ in
         [ darwin.apple_sdk.frameworks.CoreServices ];
   };
 
+  # disable bundle install as it can't install anything in addition to what is
+  # specified in pkgs/applications/misc/jekyll/Gemfile anyway
+  jekyll = attrs: {
+    postInstall = ''
+      installPath=$(cat $out/nix-support/gem-meta/install-path)
+      sed -i $installPath/lib/jekyll/commands/new.rb -e 's@Exec.run("bundle", "install"@Exec.run("true"@'
+    '';
+  };
+
   # note that you need version >= v3.16.14.8,
   # otherwise the gem will fail to link to the libv8 binary.
   # see: https://github.com/cowboyd/libv8/pull/161