refetch orphaned gh-pages branch in workflow

Author: trevorcampbell

.github/workflows/update_book.yml

@@ -33,6 +33,11 @@ jobs:
           force_orphan: true # this will clean up all previous PR previews / main branch preview
           cname: datasciencebook.ca
 
+      - name: checkout gh-pages
+        uses: actions/checkout@v2
+        with:
+          ref: 'gh-pages'
+
       - name: copy dev/ website back in
         run: |
           yes | cp -rf $HOME/dev dev