.github: add actionlint workflow

[ee7]

Make CI lint GitHub Actions workflows with actionlint. See the list of actionlint checks.

There isn't an official actionlint action, so I adapted the suggested usage.

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To-do:

• [ ] Consider using one of these:
  • https://github.com/reviewdog/action-actionlint
  • https://github.com/raven-actions/actionlint
  • https://github.com/devops-actions/actionlint
  • https://github.com/eifinger/actionlint-action