Sort of per: https://reproducible-builds.org/specs/source-date-epoch/

EPOCH is set to the first of:
--epoch=[seconds of UNIX epoch]
SOURCE_DATE_EPOCH from the environment
configure time seconds of UNIX epoch

The spec would prefer the last source modification for the last one,
but close enough for now.

They do not comply with the repro build spec.  Will be replaced
by EPOCH.

Another step to reproduceable build.