release: Don't try to fetch distfiles for pkgbase

In order to comply with the require that GCE images must include their
source code, we fetch distfiles for all of the packages installed into
GCE images. This fails for obvious reasons for packages with an origin
of base/*; filter those out to generate the list to fetch.

Reviewed by: ivy
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53568

(cherry picked from commit 05b3a45cd065c93cc1262d31675e3e102784400a)