Use uname -m for arch detection on Darwin family

[am11]

Currently the CI as well as my local system is showing this warning on macOS:

/Users/runner/runners/2.165.0/work/1/s/installer.sh --restore --build --ci --test -configuration Debug /p:StripSymbols=true /p:RuntimeArtifactsPath=/Users/runner/runners/2.165.0/work/1/s/artifacts/transport/coreclr /p:RuntimeConfiguration=release  /p:LibrariesConfiguration=Debug   /p:PortableBuild=true /p:SkipTests=False
========================== Starting Command Output ===========================
/bin/bash --noprofile --norc /Users/runner/runners/2.165.0/work/_temp/88dee7f6-0ef1-4d7a-ae4c-58077dd22fc0.sh
Unsupported CPU i386 detected, build might not succeed!
__DistroRid: osx-x86
__RuntimeId: osx-x86
Downloading 'https://dot.net/v1/dotnet-install.sh'
...

PR fixes the warning.

[am11]

@jkotas, I found this while investigating the failing build on other PR: https://dev.azure.com/dnceng/public/_build/results?..

[jkotas]

Related #33223

[am11]

Sorry I didn't see that PR (due to huge amount of notifications). I will close this PR.