bpo-34170: _PyCoreConfig_Read() leaves Py_IsolatedFlag unchanged (GH-8361)

* _PyCoreConfig_Read() no longer directly modifies Py_IsolatedFlag
  and Py_NoSiteFlag global configuration flags. The function now
  requires two pointers to integer, so these flags can be set later,
  to avoid side effets in _PyCoreConfig_Read().
* pathconfig_global_init() now leaves Py_IsolatedFlag and
  Py_NoSiteFlag unchanged.
* Fix pathconfig_global_init(): avoid computing the path
  configuration twice, use _PyCoreConfig_SetPathConfig().
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 325f423..584aa55 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -928,7 +928,7 @@
     config._disable_importlib = !install_importlib;
     config.install_signal_handlers = install_sigs;
 
-    err = _PyCoreConfig_Read(&config);
+    err = _PyCoreConfig_Read(&config, &Py_IsolatedFlag, &Py_NoSiteFlag);
     if (_Py_INIT_FAILED(err)) {
         goto done;
     }