Run rubocop when monorepo gems also changed

What does this MR do and why?

Run rubocop when monorepo gems also changed

Because we do run RuboCop against them. We do exclude vendor/gems on the other hand.

This is tested at !124770 (closed) where it does run rubocop and caught it: https://gitlab.com/gitlab-org/gitlab/-/jobs/4552213080

This is a corrective action for gitlab-org/quality/engineering-productivity/master-broken-incidents#2831 (comment 1448033532)

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