release: Use pkg install -f

Without -f, pkg will not write to the METALOG file if the packages
are already installed from a previous build, causing the packages to
not be included in the final install media.

Official builds start with a clean target directory and won't encounter
an issue, but this change supports iteration in development of the
release targets.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50344