debian_live_build: Reduce CPU load

During the 'mksquashfs' stage, all available CPUs are used per default.
Reduce this to 3/4 of the amount, to have more CPU power available for
other processes, e.g. openQA workers.
For osuosl3 this means a reduction from 16 to 12 cores.

Signed-off-by: Holger Levsen <holger@layer-acht.org>