Starting with Bundler 2.x deprecation warnings started appearing noting that flags will no longer persist config when running `bundle install`:

```
       Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment
       [DEPRECATED] The `--deployment` flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use `bundle config set deployment 'true'`, and stop using this flag
       [DEPRECATED] The `--path` flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use `bundle config set path 'vendor/bundle'`, and stop using this flag
       [DEPRECATED] The `--without` flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use `bundle config set without 'development:test'`, and stop using this flag
       [DEPRECATED] The --binstubs option will be removed in favor of `bundle binstubs`
```

This is generally a confusing part of running bundler locally where someone on a team might try running `--bundle_without` once and then be confused at future behavior.

To support persisting values, we can use the bundler env vars that start with `BUNDLE_`. These will be set at:

- Build time
  - exported for other buildpacks
- Runtime

In addition, they're being shown in the `bundle install` output so if a developer gets a failure on Heroku they can copy and paste the command to test locally.

The code makes it difficult to see what the conditions where this would be empty are. This comment makes that condition more clear.