Update action.yml

Author: sagalbot

.github/actions/setup/action.yml

@@ -11,6 +11,9 @@ runs:
         node-version: 20
         cache: 'pnpm'
 
+    - name: Install pnpm
+      run: npm install -g pnpm
+
     - uses: actions/cache@v4
       id: pnpm-cache
       with: