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.
0832addf67