tests bsim: Limit parallel build

The compile script had been modified
to fully parallelize all images builds
in 1de363d9

And has been kept building everything in parallel since then.
Over time we have seen an increase in the number of hangs
for this job in CI.
This is understood as being the result of the number of images
having increased, leading to a too high load
on the worker machines which cause them to timeout their
connection to the github CI scheduler.

In b83a8288
these jobs priority was lowered to aleviate this issue,
but again we see an increased number of CI runner
disconnects.

Let's parallelize this compile jobs a bit less.
With this change, we do not build anymore the sets of
host ll and mesh test images in parallel but sequencially.
On a local test this reduces peak load from
~1700 to ~700, and eliminates issues with system
unresponsiveness; while there is no statistically measurable
increase in overall build time.
Note that the single audio image is still built in parallel
to the other 3 sets, as this was seen to improve total
build time by 2 seconds out of 1m51s.

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