ci: Add build step for core package in React integration tests workflow

Author: exaby73

.github/workflows/react-integration-tests.yml

@@ -36,6 +36,9 @@ jobs:
       - name: Install dependencies
         run: pnpm install
 
+      - name: Build core package
+        run: pnpm --filter @firebase-ui/core build
+
       - name: Install Firebase Tools
         run: npm install -g firebase-tools