ci: Fix hierarchicalforecast downstream test (#2966)

Author: FBruzzesi

.github/workflows/downstream_tests.yml

@@ -366,7 +366,7 @@ jobs:
       - name: install-deps
         run: |
           cd hierarchicalforecast
-          uv pip install --system ".[dev]"
+          uv pip install --system ".[dev,polars]"
       - name: install-narwhals-dev
         run: |
           uv pip uninstall narwhals --system
@@ -376,7 +376,7 @@ jobs:
       - name: Run tests
         run: |
           cd hierarchicalforecast
-          nbdev_test --do_print --timing --n_workers 0
+          pytest tests
 
   formulaic:
     strategy: