Implement PR build status using check run API

[Kobzol]

After a try or an auto build is started on a PR, we should use the GitHub check run API to mark the PR as being "CI pending" (yellow dot). Similarly as homu did, although it used commit statuses instead of check runs.

• Try builds (Add try build check run #338)
• Auto builds (Merge queue #330)

[Sakib25800]

Auto builds - #330

[Kobzol]

Awesome!