use pnpm/action-setup (#15744)

Author: Rich-Harris

.github/workflows/pkg.pr.new.yml

@@ -8,10 +8,8 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - name: Checkout code
-        uses: actions/checkout@v4
-
-      - run: corepack enable
+      - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v4
       - uses: actions/setup-node@v4
         with:
           node-version: 22.x