Drop x86_64-darwin and promote the aarch64-linux CI leg - #2

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: 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/