release: Pass PKG_INSTALL_EPOCH to vmimage.subr

This value, if not already set, comes from the timestamp of the most
recent git commit (which is now also available in src/release code as
GITEPOCH) or 0 if git is not installed.

This should allow /var/db/pkg/local.sqlite to be reproducible in VM
images which have packages installed (e.g. cloudware).

Reviewed by: emaste, bapt
MFC after: 5 days
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D49760