Fix context (#28)

Author: jorgensd

.github/workflows/build_docs.yml

@@ -41,4 +41,4 @@ jobs:
         uses: peaceiris/actions-gh-pages@v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: ${PUBLISH_DIR}
+          publish_dir: ${{ env.PUBLISH_DIR }}