Updated canary workflow to use Casper 4.0 branch

no issue

- when building the canary zip, we currently use `main` within Casper,
  but the 4.0 changes are on the 4.0 branch
- this commit updates the workflow to checkout that branch as part of
  the pipeline

.github/workflows/test.yml

@@ -192,6 +192,9 @@ jobs:
       - run: yarn
       - run: grunt master
 
+      - run: git checkout 4.0
+        working-directory: content/themes/casper
+
       - run: echo "ghost_hash=$(git rev-parse --short HEAD)" >> $GITHUB_ENV
       - run: echo "ghost_admin_hash=$(git rev-parse --short HEAD)" >> $GITHUB_ENV
         working-directory: core/client