pkgbase: Unify pkg ABI handling for pkgbase targets

Right now, to get the pkg ABI we either use PKG_ABI, derived from
newvers.sh, or use an ABI file from the staged world. This
inconsistency is confusing and can cause problems.

Switch to a single source of truth: use an ABI file from the worldstage
dir to get the ABI of pkgbase packages. In particular, we do not need
to know the ABI until staging is done. More specifically:

• use a shell command to define PKG_ABI,
• replace inline uses of ABI_FILE,
• run sign-packages in a subshell (this was already done for the update-packages target) so that the staging targets are done before we try to evaluate the ABI.

Reviewed by: manu
MFC after: 1 month
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D46287

(cherry picked from commit b118b6eb4cb7520eb348a6ac965b077fc5179fde)