mkosi: simplify repo selection for rpm distros
mkosi: simplify repo selection for rpm distros

We create a custom repo file from scratch, with a bunch of repos.
We would then pass --disablerepo=* --enablerepo=… --enablerepo=…
to dnf to enable the repos we configured. This is pointlessly complicated:
let's instead just enable the repos in the config file we write.
(The config file is only used by the dnf commands we invoke.)

This changes the behaviour of UseHostRepositories=true a bit:
we now use the repos that are enabled on the host, instead of enabling
select repos. I think this actually makes more sense: the list of
repos on the host and their names is something that we have no control
over. So we don't really know which ones to enable. And the user can
still use Repositories= to select some repos. So UseHostRepositories=true
alone means "use host repositories as configured", and
UseHostRepositories=true + Repositories=… means "use the specified
host repositories".

systemd/mkosi,systemd/mkosi