Ensure rspec:feature-flags run on MR pipelines

Ensure rspec:feature-flags run on MR pipelines to catch unused feature flags before reaching master.

Prevent #284595 (closed)

This turns out to be more complex than expected.

Simply running the job in MR pipelines as attempted in !47991 (closed) would not be sufficient. The script that checks feature flags depends on artifacts generated by rspec tests that were run in the pipeline.

The script checks through all defined feature flags in the code, and verifies that the feature flags have been used in tests. Given that we may not run all rspec levels in a pipelines (based on rspec test level, frontend changes, etc), the used-feature-flags script may fail.