Re-activate Notebook validation

In order to push notebooks with a commented load stmt. - s.t. participants don't accidentally see the solution - the validation was deactivated in a589d85. However, it is a useful check to not hand out notebooks with solutions included. Therefore, it has to be re-activated with a nice workaround s.t. the `%load` magic command not executed by accident.

```bash
- name: validate notebooks
run: |
for notebook in notebooks/*.ipynb; do
if grep -q '# *%load ' "$notebook"; then
echo "$notebook contains an executed load statement.
This is not allowed inside the docker image for participants, make sure you remove this
in a future commit."
exit 255
fi
done
```