mkosi: Disable selinux labelling and install policy in initramfs

mkosi: Disable selinux labelling and install policy in initramfs

It is necessary to install the selinux policy in the initramfs
so that userland is entered with the correct label.

SELinuxRelabel defaults to auto, which will skip if the relabelling
command is not installed and will treat failure to relabel as non-fatal.

We can't force it on because root privileges are required if the labels
don't exist on the host system and we would like to be able to
cross-build from other distributions.

Since we are already committed to relabelling on first boot
there is no value in even trying to label.