Proof of concept for GitHub Actions

[callahad]

Signed-off-by: Dan Callahan danc@element.io

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
• Pull request includes a sign off
• Code style is correct (run the linters)

[callahad]

Closing in favor of #9661; apparently PRs from external repos which define entirely new Actions workflow files don't automatically run themselves :)