This allows us to add CI for CentOS Stream 10 as EPEL 10 doesn't
exist yet and won't exist for quite some time.
CentOS Stream 10 will be enabled later as soon as
https://issues.redhat.com/browse/RHEL-46604 is resolved.
We want the exitrd image to be built with the latest systemd as well.
As the exitrd image is built as part of mkosi.images, and all subimages
are built before the main image, this implies the packages must be built
as a subimage in mkosi.images/ as well. So we introduce the build image and
move all logic related to building distribution packages there.
This also has the nice side effect of slimming down the main image as the
build dependencies are not installed into the main image anymore. It also
makes sure the packages are built in a "clean" chroot without any of the
other packages which we install in the main image available.
