flake: use systems

Make the flake externally extensible. Remove flake-utils to keep the
dependency tree lean.

flake.nix

@@ -3,29 +3,24 @@
   # To update all inputs:
   # $ nix flake update --recreate-lock-file
   inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
-  inputs.flake-utils.url = "github:numtide/flake-utils";
+  inputs.systems.url = "github:nix-systems/default";
 
-  outputs = inputs:
-    inputs.flake-utils.lib.eachSystem [
-      "aarch64-darwin"
-      "aarch64-linux"
-      "i686-linux"
-      "x86_64-darwin"
-      "x86_64-linux"
-    ]
-      (system:
-        let
-          pkgs = import inputs.self {
+  outputs = { self, nixpkgs, systems }:
+    let
+      eachSystem = nixpkgs.lib.genAttrs (import systems);
+    in
+    {
+      legacyPackages = eachSystem (system:
+          import self {
             inherit system;
             inputs = null;
-            nixpkgs = inputs.nixpkgs.legacyPackages.${system};
-          };
-        in
-        {
-          legacyPackages = pkgs;
-          devShells.default = pkgs.fromTOML ./devshell.toml;
-        }
-      ) // {
+            nixpkgs = nixpkgs.legacyPackages.${system};
+          }
+      );
+
+      devShells = eachSystem (system: {
+        default = self.legacyPackages.${system}.fromTOML ./devshell.toml;
+      });
 
       templates = rec {
         toml = {