Add offline runners info in the Slack report
# What does this PR do?

So we see which runners are offline directly in the report.

Currently, this information is added only if the check is run through `check_runner_status.yml`, where all runners are checked but reported to scheduled CI channel. Adding this information avoids confusion in the case where push/doctest CI runners are offline but reported in the scheduled CI channel.


_The documentation is not available anymore as the PR was closed or merged._