chore: drop flake-utils

flake-utils' main utility is making flakes easier to make. but more
modernly they don't provide much utility above of overrideable systems,
which is not being utlised here. So instead lets remove the input and
vendor our own "forAllSystems" function to recreate what we previously
used flake-utils for.

flake.nix

@@ -4,35 +4,40 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
     nix-darwin.url = "github:LnL7/nix-darwin";
-    flake-utils.url = "github:numtide/flake-utils";
     brew-src = {
       url = "github:Homebrew/brew/4.4.5";
       flake = false;
     };
   };
 
-  outputs = { self, nixpkgs, nix-darwin, flake-utils, brew-src, ... } @ inputs: let
+  outputs = { self, nixpkgs, nix-darwin, brew-src, ... } @ inputs: let
     # System types to support.
     supportedSystems = [ "x86_64-darwin" "aarch64-darwin" ];
 
     flakeLock = builtins.fromJSON (builtins.readFile ./flake.lock);
     brewVersion = flakeLock.nodes.brew-src.original.ref;
-  in flake-utils.lib.eachSystem supportedSystems (system: let
-    pkgs = nixpkgs.legacyPackages.${system};
+
+    forAllSystems =
+      function:
+      nixpkgs.lib.genAttrs supportedSystems (
+        system: function nixpkgs.legacyPackages.${system}
+      );
   in {
-    packages = pkgs.callPackage ./pkgs {
+    packages = forAllSystems (pkgs: pkgs.callPackage ./pkgs {
       inherit inputs;
-    };
-    devShell = pkgs.mkShell {
+    });
+
+    devShell = forAllSystems (pkgs: pkgs.mkShell {
       nativeBuildInputs = with pkgs; [
       ];
 
       BREW_SRC = brew-src;
-    };
-    ci = import ./ci (inputs // {
-      inherit pkgs;
     });
-  }) // {
+
+    ci = forAllSystems (pkgs: import ./ci (inputs // {
+      inherit pkgs;
+    }));
+
     darwinModules = {
       nix-homebrew = { lib, ... }: {
         imports = [