release: use pkg install -f

[ifreund_freebsdfoundation.org]

Summary

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.

Sponsored by: The FreeBSD Foundation

[ifreund_freebsdfoundation.org]

Git branch: https://github.com/ifreund/freebsd-src/tree/bsdinstall-offline-pkgbase

[emaste]

The release artifact targets have some problems with re-running, and we should really either be able to reuse the previously staged package install, or forcibly start with a clean target location. That said, the official builds are always done in a fresh/clean directory and won't encounter issues of this sort, and there won't be any potential downside from this change there. So I'm happy with this to aid with iteration in developing the release targets.