ci: fail build if asset bundling fails (#27820)

diff --git a/.github/workflows/ui-tests.yml b/.github/workflows/ui-tests.yml
index 658892c..d765f04 100644
--- a/.github/workflows/ui-tests.yml
+++ b/.github/workflows/ui-tests.yml
@@ -104,6 +104,8 @@
 
       - name: Build Assets
         run: cd ~/frappe-bench/ && bench build
+        env:
+          CI: Yes
 
       - name: UI Tests
         run: cd ~/frappe-bench/ && bench --site test_site run-ui-tests erpnext --headless