github-actions: readding previously removed "-n auto"

possibly this is fixed by the wheel dependency that was missing before.

src/setup.cfg

@@ -3,7 +3,7 @@ exclude = migrations, paperless/settings.py, .tox, */tests/*
 
 [tool:pytest]
 DJANGO_SETTINGS_MODULE=paperless.settings
-addopts = --pythonwarnings=all --cov --cov-report=html
+addopts = --pythonwarnings=all --cov --cov-report=html -n auto
 env =
   PAPERLESS_DISABLE_DBHANDLER=true