reproducible debian: setup_pbuilder: use [check-valid-until=no] instead of...

reproducible debian: setup_pbuilder: use [check-valid-until=no] instead of Acquire::Check-Valid-Until

this way src:debian-installer will pick up the setting as well while
building, and know to ignore Release expiration.  Otherwise, the chnage
shold be logically equivalent anyway for our uses.

(applied again bbe0f1b2 after it was
reverted)

Signed-off-by: Mattia Rizzolo <mattia@debian.org>