reproducible debian: simplify conditions

* at that point we already know that SAVE_ARTIFACTS=0 (well we know != 1, but
  0 is the only other value)
* avoid nested if while still checkin for the same value

pondering a `case` statement here…

Signed-off-by: Mattia Rizzolo <mattia@debian.org>
Signed-off-by: Holger Levsen <holger@layer-acht.org>