ci: specify pnpm version

Author: yyx990803

Diff for: .github/workflows/ci.yml

@@ -14,6 +14,8 @@ jobs:
 
       - name: Install pnpm
         uses: pnpm/action-setup@v2
+        with:
+          version: 7
 
       - name: Set node version to 16
         uses: actions/setup-node@v2