I'm a little confused. I'm trying to use writeShellScript in a flake output, but I'm being told that the writeShellScript attribute is missing. Here's an excerpt from my flake.nix, any ideas?

{
  inputs = {
    flake-utils.url = "github:numtide/flake-utils/v1.0.0";
    nixpkgs.url = "github:nixos/nixpkgs/nixos-25.11";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        vm = nixpkgs.lib.nixosSystem { ... }
        vmScript = nixpkgs.lib.writeShellScript "run-vm.sh" ''...'';
      in ...
}