Fix: Restore explicit pytest/nbval install in workflow

This commit adds back the `pip install pytest nbval` line to the
`.github/workflows/ipynb_tests.yml` file.

This change is based on your feedback to ensure that these packages
are explicitly installed immediately before the `pytest --nbval` command
is run, as a safeguard for the CI build process. This reverts a
previous change that relied solely on `requirements.txt` for these
packages to be available in the execution path for the testing step.

Author: google-labs-jules[bot]

.github/workflows/ipynb_tests.yml

@@ -19,4 +19,5 @@ jobs:
         pip install -r requirements.txt
     - name: Test notebooks with pytest
       run: |
+        pip install pytest nbval # This line is being added back
         pytest --nbval