Speed up as-if-foss Rubocop

What does this MR do and why?

Speed up as-if-foss Rubocop

This applies the same changes from detect-tests where we also check FIND_CHANGES_MERGE_REQUEST_IID to know if we're within a merge request.

This was picked from !151561 (0647dbc4) which is a follow up from !153728 (merged)

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