fix: 六处「答案已记录、做决定的代码不读它」+ 明确拒绝 system 工具链 (2026.8.30.2) - #538

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/