feat: Replace flake-utils input with plain nixpkgs.lib.genAttrs

[sestrella]

The eachDefaultSystem function from flake-utils can be replaced with the following code snippet:

systems = [
  "aarch64-darwin"
  "aarch64-linux"
  "x86_64-darwin"
  "x86_64-linux"
];
forAllSystems = nixpkgs.lib.genAttrs systems;

This reduces the dependency footprint for the Nix flake, as it is explained in the following article:

nixpkgs.lib.genAttrs offers a more direct, built-in alternative. Using it with an explicit list of systems gives you clear control without an extra dependency.

Source: https://nixcademy.com/posts/1000-instances-of-flake-utils/

Additionally, nixpkgs.legacyPackages.${system} is used in favor of import nixpkgs { inherit system; } to avoid creating too many instances of nixpkgs as it is explained in the following article:

Instead of instantiating a new nixpkgs, access nixpkgs.legacyPackages.$ and then make sure that all dependencies use the same instance of nixpkgs.

Source: https://zimbatm.com/notes/1000-instances-of-nixpkgs/