Make sure that user-site does not get precedent over conda-installed packages

From @mnoethe at Slack

When having packages in $HOME/.local/lib/pythonx.y/site-packages (e.g. by doing pip install --user ...) these packages will get used even in conda environments.

This can be prevented by export PYTHONNOUSERSITE=1 (for the whole session) which we should probably do for all jobs to make sure they only use the packages in the conda / virtual env for that production.