test _build folder and commands

Author: mmcky

.github/workflows/ci.yml

@@ -44,6 +44,10 @@ jobs:
       - name: Build HTML
         shell: bash -l {0}
         run: |
+          ls lectures/
+          ls lectures/_build/
+          ls lectures/_build/jupyter_execute
+          jb build lectures
           jb build lectures --path-output ./ -n -W --keep-going
       # Build Assets (Download Notebooks and PDF via LaTeX)
       - name: Build Download Notebooks (sphinx-tojupyter)