mkosi: Add centos_epel config

mkosi: Add centos_epel config

Now that mkosi has centos-stream 9 support, let's add a config in
the repo so that the mkosi CI tests that configuration as well.

Centos doesn't support btrfs so we use xfs instead. For some reason,
building --hostonly-initrd centos images breaks the qemu boot so I
disabled that option for centos.

We update the mkosi commit hash to https://github.com/systemd/mkosi/commit/0dd39c20a4b3a2fab6efdc54da92bffad7c7b7ca
which adds the PowerTools repo to CentOS Stream 8 which is required
to make all the necessary packages required to build systemd on
CentOS Stream 8 available.