[3.12] gh-110932: Fix regrtest for SOURCE_DATE_EPOCH (GH-111143) (#111154)

gh-110932: Fix regrtest for SOURCE_DATE_EPOCH (GH-111143)

If the SOURCE_DATE_EPOCH environment variable is defined, use its
value as the random seed.
(cherry picked from commit 7237fb578dc9db9dc557759a24d8083425107b91)

Co-authored-by: Victor Stinner <vstinner@python.org>