GitHub Actions does not offer any way to determine the pull request number in a machine readable way from the checks API. See #27178 (comment).
However, the pull request number can be useful for external tools to act on CI results.
Fix that by using a check run annotation for a single task named debug_pull_request_number_str.
This should re-enable the ‘CI Failed’ labelling mechanism via https://github.com/maflcko/DrahtBot/commit/1f24cc1ab9be8ad35fbb3a44aaa073bf669a7685.