Create branch first.

.github/workflows/ci.yml

@@ -45,6 +45,7 @@ jobs:
         # Only run on main branch push (e.g. pull request merge).
         if: github.event_name == 'push'
         run: |
+          git checkout -b deploy
           git config --global user.name "Deployer"
           git config --global user.email "username@users.noreply.github.com"
           git add --force book/book