revert testing endpoint

Author: tsmathis

.github/workflows/testing.yml

@@ -57,7 +57,7 @@ jobs:
       - name: Test with pytest
         env:
           MP_API_KEY: ${{ secrets[env.API_KEY_NAME] }}
-          MP_API_ENDPOINT: https://api-preview.materialsproject.org/
+          # MP_API_ENDPOINT: https://api-preview.materialsproject.org/
         run: |
           pip install -e .
           pytest -n auto -x --cov=mp_api --cov-report=xml