fix: Update GitHub Actions workflow to set PYTHONPATH

This change ensures that the pytest command can find the `streak` module during the CI run.

Author: google-labs-jules[bot]

.github/workflows/pytest.yml

@@ -17,4 +17,4 @@ jobs:
         pip install pytest
     - name: Test with pytest
       run: |
-        pytest
+        PYTHONPATH=. pytest