release: Install pkg from the pkgbase repo

If we're building pkgbase media, install the pkg package from the
pkgbase repository we already created, not from pkg.freebsd.org.
Aside from being more desirable, this also fixes make release for
version/architecture combinations which are missing repository
on pkg.f.o (which is nearly all of them at the time of writing).

This doesn't yet work for bootonly, since we don't build the pkgbase
repository there. For now, restore the old behaviour of attempting
to install pkg from pkg.freebsd.org for bootonly.

MFC after: 1 day
Reviewed by: ifreund_freebsdfoundation.org, cperciva (previous version)
Differential Revision: https://reviews.freebsd.org/D52637