#54827: ci: use Cloudflare DNS directly on macOS runners
Description of Change
The macOS runner VMs get 192.168.64.1 from DHCP as their only resolver. That is a caching, UDP-only DNS proxy on the VM host which forwards to Google Public DNS, so TCP DNS fails and answers are re-synthesized by the proxy.
This makes the first step of every macOS job (build, publish, test, gn-check, clang-tidy, pgo-collect and macos-disk-cleanup) point the system resolver at 1.1.1.1/1.0.0.1 and flush the DNS cache, before checkout. No restart is needed. It is an inline run: step rather than a composite action because a local action is not available until after checkout.
Probed on macos-15-large and macos-15-xlarge in this run: after the switch, lookups go to Cloudflare (AS13335), TCP DNS works and the setting persists.
Checklist
- PR description included
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Backports
Semver Impact
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- Major (X.y.z): Breaking changes that may require code modifications
- Minor (x.Y.z): New features that maintain backward compatibility
- Patch (x.y.Z): Bug fixes that don't change the API
- None: Changes that don't affect using facing parts of Electron