Fix missing permissions

actions/jekyll-build-pages is a Docker action hence is run by the default Docker
user (root). So we need sudo in copying into the directory created by Jekyll.
See actions/jekyll-build-pages#101.

https://docs.github.com/en/actions/creating-actions/dockerfile-support-for-github-actions#user

.github/workflows/jekyll-gh-pages.yml

@@ -47,7 +47,7 @@ jobs:
       - name: Documentation
         run: l3build doc -q -H --show-log-on-error
       - name: Copy PDF
-        run: pwd; ls; cp *.pdf ./_site/ 
+        run: sudo cp *.pdf ./_site/
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v3