fix(build): set target_arch to fix NSS build on Apple Silicon

[groovecoder]

GYP expects the target_arch environment variable to determine the correct architecture when building NSS. On Apple Silicon, if this is unset or misconfigured, NSS can be built for x86_64, leading to linker failures due to architecture mismatch.

This patch sets target_arch based on uname -m when building on macOS, ensuring correct architecture selection and avoiding these build errors.

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes
• Quality: This PR builds and tests run cleanly
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  • I'm getting these test failures, but they seem unrelated to this change:

failures:
    selector::tests::test_filter_with_remote_settings
    selector::tests::test_filter_with_remote_settings_overrides
    selector::tests::test_remote_settings_empty_search_config_overrides_filtered_without_error
    selector::tests::test_remote_settings_empty_search_config_records_throws_error
    selector::tests::test_remote_settings_search_config_overrides_records_is_none_throws_error

test result: FAILED. 40 passed; 5 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.04s

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