Add pnpm setup step in GitHub Actions workflow

Author: fallow64

.github/workflows/nextjs.yml

@@ -36,6 +36,8 @@ jobs:
         with:
           node-version: "22"
           cache: 'pnpm'
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
       - name: Install dependencies
         working-directory: ./website
         run: pnpm install