Bootstrap pkg before building a pkg package

At sufficiently high levels of build parallelism we can try to create
the pkg before any of the other pkgbase-creation steps have taken
care of the bootstrap.

Fixes: 6b5adf33873a ("Include a pkg package in the pkgbase repo")
MFC after: 12 hours
Sponsored by: https://www.patreon.com/cperciva

(cherry picked from commit 98d74493619ee0e1587bffed79ce8e9573c3fc8b)