add exit code check to soroban-examples diff step

Author: leighmcculloch

.github/workflows/test-with-soroban-examples.yml

@@ -75,7 +75,7 @@ jobs:
         done
 
     - name: Diff
-      run: git diff
+      run: git diff --exit-code && (echo 'A diff is expected'; exit 1) || true
 
     - name: Build soroban-examples
       env: