deps: bump actions/checkout from 6 to 7 - #12

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: DeterminateSystems/flake-checker-action@v12, DeterminateSystems/nix-installer-action@v21. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/