Installed disk still shows the live installer banner in /etc/issue #61

After a disk install finishes, the installed system's login screen still reads:

    Installing Alpine.
(*  =  *)  Unattended. Powers off when done.
           Do not log in.
           Watch progress: stoat logs <vm>

The banner belongs to the live ISO's apkovl overlay and tells the user not to log in. setup-disk -m sys copies the running root's /etc into the target, so /etc/issue goes with it. It is cosmetic and it is wrong on a finished install.

Reproduce with stoat create X --image alpine-standard --mode disk, stoat up X, and look at the console after the post-install boot.

The install script should restore or replace /etc/issue on the target before powering off. Related: #39, which stopped the install script itself from re-running on the installed disk.

Found while clearing the needs-live-boot debt on #48.