pkgbase-stage.lua: Add pkg package to the offline repo

We need a pkg package on the media so we can install it for offline
installations; copy this package from the staging repository to the
pkgbase offline repository on the media.

MFC after: 1 day
Reviewed by: ifreund_freebsdfoundation.org, cperciva
Differential Revision: https://reviews.freebsd.org/D52636