RFC: Supply our own nproc echoing FDO_CI_CONCURRENT

Sometimes users of templates forget to use FDO_CI_CONCURRENT but invoke something like make -j$(nproc) instead. We can limit the effect of this but adding a custom nproc to our containers that simply does this:

exec echo $FDO_CI_CONCURRENT

This would need to be added to cbuild and in a PATH location of higher priority than the system one (/usr/local/bin?). Those who need the real nproc for other reasons can invoke it directly via the full path.

This won't have an effect on ninja builds which detect the CPU count programmatically.