Fix pnpm CI install and build stability

.github/workflows/build.yml

@@ -14,16 +14,17 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Install pnpm
+        uses: pnpm/action-setup@v6
+
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
           node-version: '24'
-
-      - name: Install pnpm
-        run: npm install -g pnpm
+          cache: 'pnpm'
 
       - name: Install dependencies
-        run: pnpm install
+        run: pnpm install --frozen-lockfile
 
       - name: Build Next.js app
         run: pnpm run build