bsdinstall: reset the mirror when restarting

[khorben_defora.org]

Summary

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.

Test Plan

# bsdinstall jail
[...]
[choose a mirror that doesn't work]
[...]
[choose restart]
[loop straight back into the abort dialog]

[khorben_defora.org]

This is also related to #192194.

[khorben_defora.org]

(I did not mean to remove any reviewer, I guess I re-used an old browser tab without refreshing)