Set up nix flake for building commons - #19

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache/restore@v4, actions/cache/save@v4, actions/cache@5a3ec84eff668545956fd18022155c47e93e2684. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/