Heroku doesn't support differentiated Gemfiles

I'm trying to set up dual-booting for my application, using a similar approach to the [`next_rails`](https://github.com/fastruby/next_rails) gem. Put simply, my application has a `Gemfile`/`Gemfile.lock` pair for the "stable" configuration, and a `Gemfile.next`/`Gemfile.next.lock` pair for the configuration being validated.

In my local environment, I can switch between these by setting `BUNDLE_GEMFILE=Gemfile.next` as necessary. In this way, my "next" environment can explore updates to Ruby, Rails, and other dependencies over the long-term without endangering my production app or holding up the development pipeline.

For all appearances, however, this buildpack doesn't play nicely with such a configuration. Bundler version inference, Ruby version inference, and initial gem installation are done (at least in part) based on the `Gemfile.lock` file, regardless of the value of `BUNDLE_GEMFILE`, which causes mismatches during build startup.

Start by tracing the buildpack's Bundler and Ruby version inference and initial gem installation, reproducing the build with BUNDLE_GEMFILE=Gemfile.next and its paired lockfile. Done means those steps consistently use the selected Gemfile rather than the default Gemfile.lock.

Written by the indexing model from the issue text.