Add SAGE_USE_CDNS=yes

Author: kwankyu

.github/workflows/doc-build.yml

@@ -82,6 +82,7 @@ jobs:
         # incremental docbuild may introduce broken links (inter-file references) though build succeeds
         run: |
           set -ex
+          export SAGE_USE_CDNS=yes
           mv /sage/local/share/doc/sage/html/en/.git /sage/.git-doc
           make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage
           mkdir -p /sage/local/share/doc/sage/html/en/ && mv /sage/.git-doc /sage/local/share/doc/sage/html/en/.git