Fix T73977, T73825: ignore Python user site-packages directory by default

This goes along with the existing changes to ignore PYTHONPATH by default.
--python-use-system-env now controls both.

Differential Revision: https://developer.blender.org/D6962

source/blender/python/intern/bpy_interface.c

@@ -279,9 +279,10 @@ void BPY_python_start(int argc, const char **argv)
    * While harmless, it's noisy. */
   Py_FrozenFlag = 1;
 
-  /* Only use the systems environment variables when explicitly requested.
+  /* Only use the systems environment variables and site when explicitly requested.
    * Since an incorrect 'PYTHONPATH' causes difficult to debug errors, see: T72807. */
   Py_IgnoreEnvironmentFlag = !py_use_system_env;
+  Py_NoUserSiteDirectory = !py_use_system_env;
 
   Py_Initialize();