Restore the mkosi-base image
The publish stage in 597b24f4 skipped the -base suffix for the mkosi images, so right now we're have templates like .fdo.qemu-build trying to pull an mkosi-base-... image from quay.io that doesn't exist.
This MR re-adds that suffix, together with a minor cleanup patch to make this more obvious to detect in the future, and a hacked in set of test cases to make sure that any images we try to use from the templates are actually available.