ci: bsim workflow: nice down test images build

The compile script was modified to fully parallelize
all bsim test images builds.
This speed up the build time, but as we are
building ~80 images, the average load on the system
will skyrocket to over 200.
For CI systems we are seeing in quite many cases,
the CI server loosing connection with the CI runner,
while this part of the job is run, and suspect it is
due to this load.

Nice the job down (by the default +10),
as this should alleviate this problem without slowing
down the job.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>