pkgbase: accommodate pkg < 1.17

6cafdee71d2b adapted the pkgbase build for 1.17, but broke Cirrus-CI's
use of PKG_FORMAT=tar (the quarterly package set still has pkg 1.16).
Because of this I disabled the pkgbase build and test in 2bfba2a04b05.

Now, check pkg --version and use the old logic for < 1.17.

To be reverted once we no longer encounter pkg 1.16 in Cirrus-CI (i.e.,
via GCP cloud images) to avoid keeping this extra complexity around.

PR: 257422
Reviewed by: manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31324