Build frontend assets in PR CI

The production frontend build (yarn encore production) only ran in the
release workflow on tag pushes, so build failures like the recent
ESLint one were only discovered after merging. Run it on every pull
request as well.

Author: kayjoosten

.github/workflows/test-integration.yml

@@ -29,3 +29,6 @@ jobs:
 
       - name: Run QA tests
         run: composer check-ci
+
+      - name: Build frontend assets
+        run: yarn encore production