mkosi: Set CacheOnly=metadata for test images (#40699)

The default behavior is to sync repository metadata for every image
that does not have a cache and we recently changed behavior to
invalidate
all cached images whenever we decide the repository metadata needs to
be resynced.

In systemd we have two images that are not cached because they use
BaseTrees=
hence set CacheOnly=metadata to explicitly indicate these two images
should never cause a repository metadata if resync even though they are
not cached.