Working directory of `ipa_post_processor` tool is different when builds with tree artifact outputs

According to the documentation, `ipa_post_processor` is

> A tool that edits this target's IPA output after it is assembled but before it is signed. The tool is invoked with a single command-line argument that denotes the path to a directory containing the unzipped contents of the IPA (that is, the Payload directory will be present in this directory).

But when you build with tree artifact outputs feature enabled (under the `--define=apple.experimental.tree_artifact_outputs=1` flag), the working directory of the tool is the directory containing your `.app` directory.