Bump peaceiris/actions-gh-pages from 3.7.3 to 4.0.0 (#102)

Author: dependabot[bot]

.github/workflows/build.yml

@@ -50,7 +50,7 @@ jobs:
       - name: Push to fatiando.github.io
         if: success() && ( github.event_name == 'push' || github.event_name == 'schedule' )
         # Don't use tags: https://julienrenaux.fr/2019/12/20/github-actions-security-risk/
-        uses: peaceiris/actions-gh-pages@bbdfb200618d235585ad98e965f4aafc39b4c501
+        uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e
         with:
           deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }}
           external_repository: fatiando/fatiando.github.io