I realised while doing the fix that changing sys.executable to point to the "real" python.exe would break scenarios that involve generating scripts. All of those have been relying on sys.executable launching the venv, which would break if we changed it there.

Instead, we now just load the direct executable name in multiprocessing as I posted earlier when we detect that __PYVENV_LAUNCHER__ was set on Windows. Having a "sys.base_executable" property might have been valuable here, but as we don't need it for all platforms I just used the _winapi module.