Add CI jobs to test bootupd changes with bootc-image-builder and image-builder

[travier]

Recent changes in bootupd broke functionality in https://github.com/osbuild/bootc-image-builder and https://github.com/osbuild/image-builder-cli.

As bootupd takes an increasing important place, we should try to catch those issue earlier and expand our coverage.

[supakeen]

Yep. I'll link this issue to some people but my idea would be (this applies to both bootupd and bootc):

1. build a bootable container that contains the new bootupd version.
2. use the image-builder container (from its main branch) to turn it into artifacts.

Potentially:

3. boot test the system

There are some bits and bobs lying around in the linked repositories but I need to give it a bit more thought.

Doing this would greatly reduce the risk and increase awareness between the projects which are, honestly, quite tightly coupled on their behavior :)