CI: Attempt to throttle load caused builds better

Use the support that both make and ninja have to hold back on starting new
parallel jobs if load is higher than a set number.

Use the $CI_ONIRO_NUM_CPUS variable if set, or the output of nproc command to
set sane values.

For most cases, this means that CI runners don't have to set anything, as nproc
output should be fine.

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>