Fix crash when mypy reports no errors - #599

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. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Failed to save: Unable to reserve cache with key setup-uv-2-x86_64-unknown-linux-gnu-ubuntu-24.04-3.10-9a0d4c44990fc78de4570fbde15ec32f0bb481fd4782c15330030bb06c405d24, another job may be creating this cache.