bsdinstall: reset the mirror when restarting

It is possible to restart the installation process upon errors, when
installing normally through the auto script, or when setting up a jail
with the jail script. However, some values obtained interactively from
the user or guessed by some scripts are kept in the environment when
restarting the process; this makes it impossible to run some steps as
expected after the restart.

For instance, if a bad choice of mirror was made in the mirrorselect
phase, restarting the installer remembers the choice made, and will
never prompt for a different one. Rebooting is then the only easy way
out of this situation.

This change only affects the jail script for now, as otherwise there
is no way to tell if the value had been specifically set by the user
before starting bsdinstall.

Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42183