bpo-46015: Fixes calculation of sys.path in a venv on Windows (GH-29992)

Also ensures that pybuilddir.txt is written early enough in the build to be picked up by later steps.

Modules/getpath.c

@@ -390,7 +390,7 @@ getpath_readlines(PyObject *Py_UNUSED(self), PyObject *args)
         while (cb && (p1[cb] == L'\n' || p1[cb] == L'\r')) {
             --cb;
         }
-        PyObject *u = PyUnicode_FromWideChar(p1, cb + 1);
+        PyObject *u = PyUnicode_FromWideChar(p1, cb ? cb + 1 : 0);
         if (!u || PyList_Append(r, u) < 0) {
             Py_XDECREF(u);
             Py_CLEAR(r);