system-reinstall-bootc: Automatically daemonize

I was playing around with system-reinstall-bootc targeting a remote cloud instance and my SSH connection died for some reason (network flake) and then the invocation is just off running against a detached tty.

It's the same major problem with the classic case of "ssh to server and run dnf/apt update"...I think we should automatically daemonize ourselves here via e.g. `systemd-run`.