Use BUNDLE_ONLY to load coverage gems

Context

Related to #416630 (closed)

What does this MR do and why?

Use BUNDLE_ONLY to load gems.

Before this commit, we downloaded a ruby gems cache (took ~15 seconds), and did a bundle install without most gems (took ~5 seconds).

This commit introduces the new BUNDLE_ONLY env variable to only install the specific group we'd like to use: coverage.

MR acceptance checklist

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