Allow to override build date with SOURCE_DATE_EPOCH

in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

Also use UTC to be independent of timezone.
3 jobs for date in 24 minutes and 24 seconds (queued for 5 seconds)