This commit adds an `--force-repo` flag that can be used
to replace all the base repositories with a base url to
a repository. This is useful for testing but also dangerous
as it will not do any checks and happily use a fedora-42 repository
for centos-8 depsolving.
This will make the use-case of the koji builder easier and is
also something that the `build` tool in `images` supports.
bc5be2ba8a