Python: disable user site-packages for programs and environments.

Python by default checks a `site-packages` folder in the user's home
folder. We do not want such an impurity and therefore disable it.
Fixes #26846.

pkgs/development/interpreters/python/wrap.sh

@@ -66,7 +66,9 @@ wrapPythonProgramsIn() {
                     # above. The script will set PYTHONPATH and PATH variables.!
                     # (see pkgs/build-support/setup-hooks/make-wrapper.sh)
                     local -a wrap_args=("$f"
-                                    --prefix PATH ':' "$program_PATH")
+                                    --prefix PATH ':' "$program_PATH"
+                                    --set PYTHONNOUSERSITE "true"
+                                    )
 
                     # Add any additional arguments provided by makeWrapperArgs
                     # argument to buildPythonPackage.