consider migrating to osbuild/bootc-image-builder-action for build iso

[eseiker]

https://github.com/ublue-os/bootc-image-builder-action is now stale and suggests to use https://github.com/osbuild/bootc-image-builder-action

[alexiri]

I don't have time to work on this for the time being, and it could be a relatively tricky change: I've had issues with bootc-image-builder in the past. Do you think you could tackle it, and all the testing it'll require?

[eseiker]

I think I can handle it if you share the required tests list

[eseiker]

initial draft implementation for now: https://github.com/eseiker/atomic-ci/tree/refactor-build-bootable-artifacts