certctl: invoke with LOCALBASE set

When invoking certctl as part of installworld, set LOCALBASE in the
environment to match the build environment. That ensures that LOCABASE
is non-empty on systems without the user.localbase sysctl and avoids
allowing a system configuration detail to leak into the build. Users
who wish to build targeting a non-standard LOCALBASE should ensure it is
set in src.conf or similar.

Reviewed by: Mina Galić <freebsd@igalic.co>
Differential Revision: https://reviews.freebsd.org/D40530