Set FORCE_PKG_REGISTER=1 when installing packages to avoid failures
when re-using build chroot(8) environments.

This is based on the patch in the PR referenced below, but instead
of using 'reinstall' in two locations (one of which already uses
FORCE_PKG_REGISTER=1), changes the non-embedded behavior.

PR: 205998
Submitted by: ngie
MFC after: 5 days
Sponsored by: The FreeBSD Foundation