Disable sphinx-markdown-builder in tests again for now

Somehow I didn't realize this breaks the tests/checks/sphinx-* tests.

.github/workflows/main.yml

@@ -22,7 +22,7 @@ jobs:
         sudo apt install gettext libpcre2-dev python3-pexpect python3-sphinx tmux
         # Generate a locale that uses a comma as decimal separator.
         sudo locale-gen fr_FR.UTF-8
-    - uses: ./.github/actions/install-sphinx-markdown-builder
+    # - uses: ./.github/actions/install-sphinx-markdown-builder
     - name: cmake
       run: |
         mkdir build && cd build