CI: Fail rubocop job on MRs when `RUN_ALL_RUBOCOP=true`

What does this MR do and why?

Previously, rubocop job would only silence offenses - and not fail - on MRs which potentially added them.

This behavior introduces new silenced offenses which are later reported in Slack channel via scheduled pipeline.

This commit ensures that the rubocop job fails on the MRs which introduced the offenses.

Refs

• !137876 (merged) where TODOs were regenerated
• !135806 (merged) which introduced new offenses
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/5586930222 only silenced the new offenses instead of failing

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.