There's an [issue](https://github.com/flutter/flutter/issues/138238) with mac_toolchain that makes Xcode installs flakey and an [issue](https://github.com/flutter/flutter/issues/138246) that makes Xcode installs more frequent on macOS 13, which is causing presubmit tests to fall frequently. In the meantime, we'll only have tests run on macOS 12. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style