Added the new -S flag for arch-chroot

[Torxed]

Man page for arch-chroot -S:

OPTIONS
       -S
           Run in systemd mode. This will spawn a transient systemd service via systemd-run(1) and delegates namespace/chroot management to the
           service manager.

This might introduce some minor bugs, but overall should make our lives a lot easier.
Among probably many things. this will fix/workaround the bootctl --variables issues as there was a problem with arch-chroot.

I also consolidated a lot of the individual arch-chroot calls, to use the central function. We could have combined more of them, but some of them was a bit more "one-off" and I didn't wanna touch too much in this PR.