Switch Default CI Focus to osx-arm64 for CoreCLR PRs

[am11]

In the default CI set that runs automatically when a change is made to src/coreclr, only priority 0 test legs are executed for osx-arm64. Meanwhile, for osx-x64, the libraries debug tests are run by default alongside the priority 0 tests. Given that Apple has moved to ARM64 (Apple Silicon) as the primary architecture, it seems logical to prioritize osx-arm64 in the default CI, rather than osx-x64.

Currently, we can manually trigger the osx-arm64 libraries debug tests if needed, but they are not part of the automatic/default test set.

If expanding the CI set for osx-arm64 leads to longer test cycles, it may be worth considering the trade-off between testing time and coverage, or potentially rebalancing the priority of osx-x64 tests.

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