ci: improve tox integration command

[dkrako]

This way it is easier to run tox for a single dataset like this:

tox -e integration -- tests/integration/public_dataset_processing_test.py::test_public_dataset_processing[PoTeC]

The command should be added to the CONTRIBUTING.md (#692)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (721d699) to head (7ad8bdc).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #693 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 55 55 Lines 2463 2463 Branches 621 621 ========================================= Hits 2463 2463 ```

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