lefthook: Run RuboCop in pre-push as well

What does this MR do and why?

Some developers tend to disable pre-commit hooks to speed up their flow so having RuboCop run in pre-push (as a safety net) is crucial.

Refs !127376 (comment 1501398064)

How to set up and validate locally

1. Check out this branch
2. Add a misplaced newline in lib/backup.rb, e.g. right after module Backup
3. Commit the change skipping lefthook: LEFTHOOK=0 git commit -m "Just testing" lib/backup.rb
4. Run lefthook run pre-push
5. See 👇

$ be lefthook run pre-push
Lefthook v1.4.7
RUNNING HOOK: pre-push
merge_conflicts: (skip) settings

  EXECUTE > security_harness

static-verification: (skip) settings
eslint: (skip) no files for inspection
db-schema-changes: (skip) no files for inspection
docs-trailing_spaces: (skip) no files for inspection
audit_event_types_docs: (skip) no files for inspection
graphql_docs: (skip) no files for inspection
docs-deprecations: (skip) no files for inspection
prettier: (skip) no files for inspection
docs-metadata: (skip) no files for inspection
markdownlint: (skip) no files for inspection
jsonlint: (skip) no files for inspection
haml-lint: (skip) no files for inspection
stylelint: (skip) no files for inspection
sidekiq-queues: (skip) no files for inspection
gettext: (skip) no files for inspection
vale: (skip) no files for inspection

  EXECUTE > yamllint

bundler-checksum-lint: (skip) no matching push files
danger: (skip) no matching push files

  EXECUTE > rubocop
Inspecting 1 file
C

Offenses:

lib/backup.rb:4:1: C: [Correctable] Layout/EmptyLinesAroundModuleBody: Extra empty line detected at module body beginning.

1 file inspected, 1 offense detected, 1 offense autocorrectable


SUMMARY: (done in 2.54 seconds)
✔️  security_harness
✔️  yamllint
🥊  rubocop

MR acceptance checklist

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