Allow customisation of max-jobs instead hardcore to a maximum of 8
Or, as a minimum, bump the max to something else bigger than 8
This has probed to make things slower in machines with a lot of cores (96) but relatively slow, like our aarch64 runners
https://gitlab.com/BuildStream/buildstream/blob/master/src/buildstream/_project.py#L808
Edited by Javier Jardón