fix for CI fix

Author: codemasher

.github/workflows/ci.yml

@@ -132,6 +132,9 @@ jobs:
     if: github.ref_name == 'main'
     runs-on: ubuntu-latest
 
+    env:
+      SPHINXBUILD: ci-venv/bin/sphinx-build
+
     steps:
       - name: "Checkout sources"
         uses: actions/checkout@v4