Fix the version specifier for the new p4python arm64 builds.
Review Request #12023 — Created Jan. 31, 2022 and submitted
My previous change to include our custom p4python arm64 builds wasn't
right. The wrong specifier was being used to match arm64. I've fixed it
platform_system, and made that
conditional on running on Darwin.
Tested a clean installation. Got the right builds on M1.