Update GitHub Actions + use cache - #152

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: py-actions/flake8@84ec6726560b6d5bd68f2a5bed83d62b52bb50ba. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/