ci: find Homebrew in macOS notarization proof

[Jesssullivan]

Summary

• make the macOS FileProvider package notarization proof tolerate self-hosted runners where Homebrew is installed but not on PATH
• skip Homebrew install when protoc is already available
• emit a hard error only when neither protoc nor Homebrew can be found

Verification

• git diff --check

Tracking

• TIN-1547 diagnostic package build for ci: probe FileProvider root from signed HostApp #405
• failed run 26114644177 stopped at brew: command not found on PZM