Track Nixpkgs stable
The Nix build fails on Nixpkgs 24.11 due to CGO_ENABLED being renamed to env.CGO_ENABLED https://github.com/NixOS/nixpkgs/commit/3b1ec755ca88778d4b706b6dd52b57c0086b1548
This PR changed the Nix flake Nixpkgs input to 24.11 instead of the master branch, which makes it easier to deploy with systems tracking stable Nixpkgs.