Bootc distro requires root filesystem even if `disk.yaml` exists

[jbtrystram]

When running image-builder-cli on an image that have a complete /usr/lib/image-builder/bootc/disk.yaml we should not have to provide a default root filesystem at the bootc level, because bootc will be called with to-filesystem and it's not initializing it.

Right now, we hit the following:

Manifest generation step
Building manifest for bootc-based-qcow2
error: failed to initialize bootc distro: missing required info: DefaultRootFs

Even though our disk.yaml is providing all the required information.

See discussion in coreos/fedora-coreos-config#4194

[supakeen]

The default root fs is sourced from the install configuration of bootc itself; we get it by executing bootc install print-configuration in the container.

It is indeed a bit weird that this would be defined in both locations.

[joelcapitao]

I've proposed the first PR #2405 and a next one will come in osbuild/image-builder-cli