[EE] Add a 'custom cop' to RuboCop that bans ENV assignment
What does this MR do?
EE MR for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14810
Does this MR meet the acceptance criteria?
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Tests added for this feature/bug - Review
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Has been reviewed by Backend
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Conform by the style guides -
Squashed related commits together