Stub binary doesn't support `sys.executable` spawn/multiprocessing

### Describe the bug

The stub binary used to start an app is an effective replacement for the `python` binary; however, it isn't a perfect replacements. `multiprocessing` in spawn mode will try to invoke `sys.executable` to spawn a child; it's also common to use `subprocess` or `os.spawn` to try and start `sys.exectuable` to run other Python code as a subprocess. However, the stub binary doesn't support this kind of execution.

### Steps to reproduce

1. Generate a stub macOS app
2. Add a call to call `multiprocessing.Process()` and start it with `.start()`

The subprocess will call the parent process entry point, and fail.

### Expected behavior

The function targeted by `Process()` should be invoked with the provided arguments.

### Screenshots

_No response_

### Environment

- Operating System: All macOS
- Python version: All
- Software versions:
- Briefcase: 0.3.12

### Logs

n/a

### Additional context

See https://github.com/beeware/briefcase-macOS-app-template/issues/7 for the original report on macOS; the fault is with the stub binary, so the fix needs to be on the Xcode template. Any fix here will be inherited by the app template.

One possible solution would be to add some logic to the stub binary to inspect to see if the parent process is "self"; if so, the execution of the app should be modified to behave as if it were a normal Python binary.

Any solution should also be ported to Linux (beeware/briefcase-linux-flatpak-template#10) and Windows (beeware/briefcase-windows-VisualStudio-template#8).