set default -j value lower on Mac for local docker builds
set default -j value lower on Mac for local docker builds

- The default was equalling number of cores on the machine, but Docker for Mac defaults to half that value (and can be adjusted via preferences)
- Set to -j2 as a safe default; edit the script if you want a higher value
- Also reversed order of the `nproc` vs `sysctl` check; a Mac with `coreutils` installed also has the `nproc` command and want the Mac branch to be used on the Mac to reduce confusion

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