Github: restart tests by comment

[andre2007]

It would be great if the checks of a pull requests can be restarted by adding a comment with the content "restart tests "

[MartinNowak]

Instead of cluttering the comments with bot-speak, it seems more reasonable to use the retry buttons in the CIs (or add them were missing). I don't think github has comment-commands like gitlab yet.
Even better would be some fancier flaky-test detection with auto-retries.
Investing in the CIs seems more worthwhile to me than programming workarounds in dlang-bot. Also this task is more complex than it might seem as we'd have to find the right build/CI to retry for the PR, likely have to check user authentication somehow, and possibly avoid abuse.