release: Publish pkgbase-repo.tar

This is a tarball containing the pkgbase repository for the release.

Note: This is an uncompressed tarball, because all of its constituent
.pkg files are already compressed; there's no point adding another
layer of compression.

Reviewed by: ivy
MFC after: 1 minute
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D52899