fix(ci): use built-in pnpm setup via setup-node@v4

Author: Alex K

.github/workflows/ci.yml

@@ -17,9 +17,7 @@ jobs:
         with:
           node-version: 20
           cache: 'pnpm'
-
-      - name: Install pnpm
-        run: npm install -g pnpm
+          package-manager: pnpm
 
       - name: Install dependencies
         run: pnpm install