fix: missed a step in openai#1171 for codex.yml (openai#1172)

Missed in my copy/paste.

Author: bolinfest

.github/workflows/codex.yml

@@ -47,6 +47,12 @@ jobs:
         with:
           node-version: 22
 
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 10.8.1
+          run_install: false
+
       - name: Get pnpm store directory
         id: pnpm-cache
         shell: bash