ci: adopt rainix nix-cachix-setup composite, drop deprecated DeterminateSystems nix installer - #357

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v2, cachix/cachix-action@ad2ddac53f961de1989924296a1f236fcfbaa4fc. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/