Take check status into account

[timokau]

Triage could automatically set to awaiting_changes and post an appropriate message when checks failed. Assigning a reviewer could be postponed until checks succeed.

https://docs.github.com/en/github/searching-for-information-on-github/searching-issues-and-pull-requests#search-by-commit-status

[timokau]

General idea:

• Subscribe to check_run.
• Reset state to awaiting_changes when a check has conclusion negative or (debatable) has conclusion Neutral and is due to a ofBorg build failure. Post a message. Suggest ways to deal with pre-existing failures (restrict platforms, mark dependencies as broken). Allow users to ignore this with a manual status override, bug suggest they try to fix the checks instead.