If no upgrades were available, the existing package needs to be
reinstalled, because the images in /boot were erased as part of the
original live CD build process, and we rely on them being present to
fix up the ISO later.

Just so that customize-inner.sh is idempotent

It takes some finagling to get this to work everywhere, but this seems
to do the trick.  Note that this means we need to set
UBIQUITY_AUTOMATIC=1, which has some other side effects, but nothing
too bad (mostly related to not being able to go backwards in the
ubiquity installer).

Also regenerate SSH keys at LiveCD boot, but not at install, so that
they don't change after booting a live system.