Nix expression build example for Ruby

[dcarley]

Acceptance criteria

Create a Nix expression build example for Ruby.

Why

So that we have examples for all languages that users can copy from.

Notes

This was deferred from flox/flox#3339 because I couldn't get it working due to:

• Gemfile.lock needs converting to gemset.nix with bundix (docs) and I'm not sure how we'd keep them in-sync
• bundlerApp (docs) is the idiomatic way to package an app with a executable but it appears to require that the app is converted to a Gem itself and referenced with a separate Gemfile
• pname and exe need to have a -nix suffix but this may not be compatible with a single gemspec for the other build types