Update action.yml

Author: sagalbot

.github/actions/setup/action.yml

@@ -9,7 +9,10 @@ runs:
     - uses: actions/setup-node@v4
       with:
         node-version: 20
-        cache: pnpm
+        cache: 'pnpm'
+
+    - name: Install pnpm
+      run: npm install -g pnpm
 
     - uses: actions/cache@v4
       id: pnpm-cache