installworld: run certctl rehash after installation completes

This was originally introduced back in r360833, and subsequently reverted
because it was broken for -DNO_ROOT builds and it may not have been the
correct place for it.

While debatably this may still not be 'the correct place,' it's much cleaner
than scattering rehashes all throughout the tree. brooks has fixed the issue
with -DNO_ROOT by properly writing to the METALOG in r361397.

Do note that this is different than what was originally committed; brooks
had revisions in D24932 that made it actually use the revised unprivileged
mode and write to METALOG, along with being a little more friendly to
foreign crossbuilds and just using the certctl in-tree.

With this change, I believe we should now have a populated /etc/ssl/certs in
the VM images.

MFC after: 1 week