ci: Switch GitHub Actions workflow from npm to pnpm.

Author: jianyuan

.github/workflows/build.yml

@@ -13,9 +13,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-node@v4
+      - uses: pnpm/action-setup@v4
         with:
-          node-version: '22'
-          cache: npm
-      - run: npm install
-      - run: npm run build
+          cache: true
+      - run: pnpm install
+      - run: pnpm run build