release: Fail the build if pkg can't be installed

If we can't install pkg(8) on the media, instead of ignoring the error,
fail the build. This avoids silently creating media which doesn't have
pkg(8) on, and therefore can't be used for a pkgbase install.

Installing pkg(8) can still be entirely disabled by setting NOPKG.

MFC after: 1 day
Reviewed by: ifreund_freebsdfoundation.org, cperciva
Differential Revision: https://reviews.freebsd.org/D52635