Use concurrency for GitHub Actions workflow

[ydah]

Same as: rubocop/rubocop-rspec#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:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• [-] Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• [-] Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [-] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.