build: Fix recent commit for SOURCE_DATE_EPOCH.

which was recently pushed to make use of $SOURCE_DATE_EPOCH
as fallback.

(cherry picked from commit 61bb75d045a3709d1cba0084c95e991dfd52c8ee)

• Fixes-commit: 5ade2b68db231c78d8ecca0eb21db2153da958d2
• Signed-off-by: Werner Koch <wk@gnupg.org>