Adopt nix-systems pattern

Somewhat described at https://github.com/nix-systems/nix-systems

This is useful for Haskell projects, because commands like

```bash
nix flake show --allow-import-from-derivation
```

can often fail, complaining that it can’t find some other architecture
to build on. With nix-systems, you can append
`--override-input systems github:nix-systems/x86_64-linux` to the
command to ensure that x86_64-linux is the only platform in play.

flake.nix

@@ -7,22 +7,26 @@
   };
 
   inputs = {
+    flake-utils = {
+      inputs.systems.follows = "systems";
+      url = "github:numtide/flake-utils";
+    };
     haskellNix.url = "github:input-output-hk/haskell.nix";
     nixpkgs.follows = "haskellNix/nixpkgs-2405";
-    flake-utils.url = "github:numtide/flake-utils";
+    systems = {
+      flake = false;
+      url = "./nix/systems.nix";
+    };
   };
 
   outputs = {
-    self,
+    flake-utils,
     haskellNix,
     nixpkgs,
-    flake-utils,
+    self,
+    systems,
   }:
-    flake-utils.lib.eachSystem [
-      "x86_64-linux"
-      "x86_64-darwin"
-      "aarch64-darwin"
-    ]
+    flake-utils.lib.eachSystem (import systems)
     (system: let
       ## It’s much easier to read from a JSON file than to have JSON import from some other file, so we extract some
       ## configuration from the VS Code settings to avoid duplication.

nix/systems.nix

@@ -0,0 +1,4 @@
+[ "aarch64-darwin"
+  "x86_64-darwin"
+  "x86_64-linux"
+]