Update shell-tool-mcp.yml (#10092)

## Summary
Remove pnpm version so we rely on package.json instead, and fix the
mismatch due to https://github.com/openai/codex/pull/9992

.github/workflows/shell-tool-mcp.yml

@@ -282,7 +282,6 @@ jobs:
       - name: Setup pnpm
         uses: pnpm/action-setup@v4
         with:
-          version: 10.8.1
           run_install: false
 
       - name: Setup Node.js
@@ -378,7 +377,6 @@ jobs:
       - name: Setup pnpm
         uses: pnpm/action-setup@v4
         with:
-          version: 10.8.1
           run_install: false
 
       - name: Setup Node.js