[EE] Add a 'custom cop' to RuboCop that bans ENV assignment (!3170) · Merge requests · GitLab.org / GitLab · GitLab

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?

Tests added for this feature/bug
Review
- Has been reviewed by Backend
Conform by the style guides
Squashed related commits together