Improve macOS CI warnings and add system LibreSSL build coverage

[daviduhden]

Summary

This PR improves macOS CI by reducing workflow warnings and adding a dedicated build path that links against the system LibreSSL stack.

What changes

• updates .github/workflows/build-macos.yml
• keeps existing Homebrew-based build job
• adds a second job to build with system LibreSSL via CMake
• reduces noisy workflow warnings by:
  • pinning actions/checkout to current runtime-compatible commit
  • disabling credential persistence in checkout
  • avoiding unnecessary Homebrew auto-update/install noise where possible
  • suppressing CMake developer/deprecation warning output in CI invocation
• keeps workflow permissions minimal (contents: read)

Why this is better

• validates LibreSSL behavior on macOS explicitly, not only Homebrew OpenSSL
• improves CI signal quality by removing avoidable workflow warnings
• preserves existing build coverage while adding a production-relevant TLS variant