reproducible debian: _build.sh: don't exec abort.sh anymore

the job is not run by jenkins anymore, so no point in calling abort.sh,
futhremore `exec`ing doesn't run the cleanup part in some cases, causing
stale temporary directories to be left behind.

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