Use make -j8 for macOS

[hugovk]

Follow on from #1541.

If a macOS user does not have coreutils installed (for example, via Homebrew) then $(nproc) can fail:

❯ make -j $(nproc)
zsh: command not found: nproc
./python.exe -E ./Tools/build/generate-build-details.py `cat pybuilddir.txt`/build-details.json
Checked 113 modules (35 built-in, 78 shared, 0 n/a on macosx-15.4-arm64, 0 disabled, 0 missing, 0 failed on import)

We want these instructions to work without further assistance for a beginner.

And I do have coreutils installed on my other machine, but not on this one.

j8 seems a reasonable number for macOS, all new Macs seem to be at least 10, and the M1 was at least 8.

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