Replace failing non-required checks to warning messages

[achilleas-k]

We have a few non-required tests in this repository currently. Let's ignore cases where a check is temporarily removed from the required list because of a known issue that's expected to be resolved¹. The reverse-dependency / integration tests with osbuild-composer are not meant to block a PR from merging, but to notify a PR author and its reviewers that the changes break compatibility with osbuild-composer and that some work will be required to update the images dependency. This can happen quite frequently, which means that a lot of times, it becomes difficult to judge, at a glance, whether a PR is ready to merge or not. It's especially annoying when looking at the PR list.

It would be much nicer if we had some automation that ran the checks and posted a comment on the PR when they failed. Bonus points if the comment only gets posted on the PR that broke the integration and not the subsequent ones. This check would have to succeed even when the integration breaks and the comment is posted. That way, the author and reviewers of a PR get notified of the breakage, they are reminded to check whether this is intentional (it could be that a change was not meant to break API compatibility, or it could make the author consider alternative implementations that don't), and the PR status would show GREEN.

An initial attempt was made to write this, but it got a bit convoluted and was ultimately reverted.

¹ For example, as of this writing, Fedora 41 unit tests are failing because our dnf5 depsolver doesn't support generating SBOMs.