See https://github.com/NixOS/nixpkgs/issues/58126 for more details.
This will instruct the user how to manually finish the setup instead of
failing with error messages (unfortunately it is quite a bit hacky
though...).
Extra note:
We cannot use "bundle config --local" due to BUNDLE_GEMFILE (would
attempt to create .bundle/config in the Nix store) and manually creating
.bundle/config doesn't work either as these configuration variables are
still overwritten by the environment variables, even though this
shouldn't be the case [0].
[0]: https://bundler.io/v2.0/man/bundle-config.1.html
d24aefd52f