Force push (with lease) the gh-pages

Because we're not keeping the history of the gh-pages branch, we end up
with a new commit every time. Github refuses that we push that branch
because it doesn't match the history.

.github/workflows/publish.yml

@@ -46,5 +46,5 @@ jobs:
             git config user.email "[email protected]"
             timestamp=$(date -u +%FT%T%z)
             ghp-import -m "Generate Pelican site ${timestamp}" --no-history --branch gh-pages  output
-            git push origin gh-pages
+            git push --force-with-lease origin gh-pages