Makefile.inc1: Conditionalize some package related variables

In particular, don't invoke git to compute SOURCE_DATE_EPOCH for
unrelated targets like check-old or delete-old. If the git invocation
fails (e.g. when using a git worktree mounted over NFS) it can
generate a lot of irrelevant warning spam.

Reviewed by: emaste
Fixes: 8a3537aaf7c1 ("Makefile.inc1: Make package timestamps reproducible by default")
Differential Revision: https://reviews.freebsd.org/D49278

(cherry picked from commit db6f2bb93a9706963f66d270edb5ee62c37a9296)