build.py: bound the make example pool by -j, not the host core count (#3950)

* build.py: bound the make example pool by -j, not the host core count

CircleCI's large class has 4 CPUs and 8 GB, but os.cpu_count() reports the
host's 36 cores. -j 4 reached only the inner make, so up to 144 compilers ran
and samd5x_e5x picks same54_xplained and d5035_01 were OOM-killed, which
CircleCI shows as a cancelled Build step with no error.