Create "All required checks succeeded" check run when "All required checks done" CI job is actually successful

[jgiannuzzi]

The "All required checks done" CI job has been designed to be a required check, but GitHub considers it successful when it's skipped, and it gets skipped when some of the jobs it depends on fail. A first attempt at fixing that was made in #366 but it was not enough.

This PR fixes that by checking in a separate workflow whether the job conclusion is success and creates a new check run "All required checks succeeded" when it's the case. When it's not the case, the check run is not created and PRs will thus not be mergeable.

Example runs:

• successful run — the check run is created by this workflow run
• skipped run — the check run was not created by this workflow run
• failed run — the check run was not created by this workflow run

I changed the workflow output slightly to show a ❌ when "All required checks done" is not successful, but this is not reflected in the above examples.