This converts all fedora links in our samples to `mirrors.kernel.org`.
This mirror works best from around the world, so lets avoid the wild
mix of local mirrors and instead use kernel.org.
This mirror is also well-managed and properly funded, so we should not
run into too many problems with it.
This is a Fedora 31 based example that results in a ostree repo
with a ostree commit. The latter was composed from the file system
tree that was created via the given osbuild stages.