Don't warn or use Gemfile.extra when not using engine_cart

[cjcolvar]

When doing a fresh checkout of hyrax and not generating the internal test app, I ran into errors running the first bundle install:

[Hyrax] Unable to find test application dependencies in /home/cjcolvar/Code/samvera/hyrax/.internal_test_app/Gemfile, using placeholder dependencies
Your Gemfile lists the gem okcomputer (>= 0) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once.
While it's not a problem now, it could cause errors if you change the version of one of them later.
Warning: the running version of Bundler (2.2.14) is older than the version that created the lockfile (2.2.17). We suggest you to upgrade to the version that created the lockfile by running `gem install bundler:2.2.17`.
Fetching gem metadata from https://rubygems.org/........
Could not find gem 'better_errors' in locally installed gems.
The source does not contain any versions of 'better_errors'

better_errors is coming from spec/test_app_templates/Gemfile.extra used by engine_cart.

Because using docker and dassie instead of engine_cart is the preferred dev environment now I think we can skip warning the user and skip trying to load Gemfile.extra when the internal test app isn't present.

@samvera/hyrax-code-reviewers

[no-reply]

this seems good.

i think you can also skip doing bundle locally on a new checkout, and skip straight to docker-compose build. but i can't see any need for this warning anymore.