ci.yml: use sysctl -n hw.logicalcpu instead of nproc on macos

[emanuele6]

The "macos (arm64)" runner on github does not have nproc.

/Users/runner/work/_temp/f44f0d9f-19eb-4a23-860e-26533d7efdfa.sh: line 10: nproc: command not found

[wader]

But it somehow worked anyway?

[emanuele6]

make -j's argument is optional in some make; e.g. in GNU make if you omit it -j it runs jobs in parallel with no limit.

Maybe macos uses GNU make?

[wader]

Aha it ended up with no stdout output on error

[emanuele6]

Yes, it would have not have worked if macos was using BSD make for example:

$ bmake -j"$(false)"
bmake: option requires an argument -- j
usage: bmake [-BeikNnqrSstWwX]
            [-C directory] [-D variable] [-d flags] [-f makefile]
            [-I directory] [-J private] [-j max_jobs] [-m directory] [-T file]
            [-V variable] [-v variable] [variable=value] [target ...]