Seems like a fairly obvious bug. From https://docs.python.org/3/c-api/init.html#c.Py_SetPath

> This also causes ... sys.prefix and sys.exec_prefix to be empty.
> It is up to the caller to modify these if required after calling
> Py_Initialize().

Apparently you can't set `sys.[exec_]prefix` before calling Py_Initialize, and you can't call Py_Initialize without setting `sys.[exec_]prefix` (much like #20891, where you can't initialize threads without holding the GIL, and you can't acquire the GIL without having initialized threads.)

I don't know how to go about resolving this though (my changes were limited to getpathp.c - and I really need to go add the same changes to the non-Windows getpath.c too...). The best way seems to be forcing Nick to finish PEP 432, but unfortunately I have no leverage over him :)