Determine how to run bundix correctly before builds

We may not want to burden people with worrying about `gemset.nix` at all, so the build now will run `bundix` on every deploy. It would be better if this step only gets run when Gemfile.lock has changed.

Locate the deploy build logic that invokes bundix and inspect how Gemfile.lock is handled. Determine how to detect changes to Gemfile.lock, then verify that bundix runs only when that file has changed and is still run during the appropriate deploy builds.

Written by the indexing model from the issue text.