Use concurrency for GitHub Actions workflow

[ydah]

Same as: https://github.com/rubocop/rubocop-rspec/pull/1539

This PR ensures that only the last execution per branch always works, even when pushed multiple times in a row, for example. The older one is canceled.

Refs: https://docs.github.com/en/actions/using-jobs/using-concurrency https://github.blog/changelog/2021-04-19-github-actions-limit-workflow-run-or-job-concurrency/

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with main (if not - rebase it).
• [x] Squashed related commits together.
• [-] Added tests.
• [-] Updated documentation.
• [-] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[pirj]

Thank you!