chore(deps): bump nix from 0.31.2 to 0.31.3 - #215

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, nix-community/cache-nix-action@v6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/