stand: Use SOURCE_DATE_EPOCH in newvers.sh

The newvers.sh script is used to generate vers.c for 10 programs (at
latest count on amd64): 4 EFI loaders, 3 x86 loaders, two userboots,
and one kboot. When building without MK_REPRODUCIBLE_BUILD this
resulted in 10 different timestamps being compiled into various files.

If SOURCE_DATE_EPOCH is set, use that value rather than the current
date and time.

Reviewed by: emaste, imp
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D48803