Python: disable user site-packages in setup hook

Wrappers already included `PYTHONNOUSERSITE=1`, but now this env var is
also set in the Python setup hook. This improves purity in case of
non-sandboxes builds and nix-shell.

pkgs/development/interpreters/python/setup-hook.sh

···
 export DETERMINISTIC_BUILD=1;
 # Determinism: We fix the hashes of str, bytes and datetime objects.
 export PYTHONHASHSEED=0;
+# Determinism. Whenever Python is included, it should not check user site-packages.
+# This option is only relevant when the sandbox is disabled.
+export PYTHONNOUSERSITE=1;