`resultBundlePath` is meant to be a directory. In the `xcodebuild --help`, it describes it as a directory: 
```
-resultBundlePath PATH     specifies the directory where a result bundle describing what occurred will be placed
```

This PR changes our usage of it from a file to a directory so that it gets deleted correctly between reruns.

Fixes https://github.com/flutter/flutter/issues/129954.