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.

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