CI: fix deploy to gh-pages branch

The `GITHUB_SECRETS` usage was causing a permissions issue,
and it seems no longer necessary.

Author: rgommers

.github/workflows/pages.yml

@@ -124,5 +124,5 @@ jobs:
       - name: 'Push changes'
         if: success()
         run: |
-          git push "https://$GITHUB_ACTOR:$GITHUB_TOKEN@github.com/${{ github.repository }}.git"
+          git push origin gh-pages
         timeout-minutes: 10