fix: force checkout of gh-pages branch (cotes2020#544)

When using the theme starter, which uses a git submodule,
assets loaded from that submodule are "untracked" files in the pages branch working tree.
The build can fail at this line due to Git, saying "the following untracked working tree files would be overwritten by checkout".

Adding `-f` forces the checkout, overwriting all files.
The other deploy steps then take over and rewrite the files as needed.

tools/deploy.sh

@@ -77,7 +77,7 @@ setup_gh() {
     _no_pages_branch=true
     git checkout -b "$PAGES_BRANCH"
   else
-    git checkout "$PAGES_BRANCH"
+    git checkout -f "$PAGES_BRANCH"
   fi
 }