bsdinstall: restore the environment when restarting

It is possible to restart the installation process upon errors, when
installing normally through the auto script, or when installing a jail
with the jail script. However, some values obtained interactively from
the user or guessed by some scripts were kept in the environment when
restarting the process; this made it impossible to re-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 remembered the choice made, and would
never prompt for a different one again. Rebooting was then the only easy
way out of this situation.

This change restores a pre-defined list of environment variables when
restarting the installation process.

PR: 266987
Reviewed by: emaste
Approved by: philip (mentor)
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D42281