release: Install pkg(8) from the release repo on bootonly

Although bootonly doesn't contain an offline package repository, we
still want pkg installed so the user can use it to repair an existing
system. Installing it from the release repository ensures it's always
available without depending on pkg.freebsd.org.

While here, fix a typo in PKGBASE_REPO_ARGS.

MFC after: 3 seconds
Reviewed by: cperciva
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52848