nix: Address warnings from "nix flake check"

Author: rvl

flake.nix

@@ -7,8 +7,22 @@
     git-ignore-nix.inputs.nixpkgs.follows = "nixpkgs";
   };
   outputs = { self, flake-utils, nixpkgs, git-ignore-nix }:
-  let
-    overlay = final: prev: {
+  flake-utils.lib.eachDefaultSystem (system: let
+    pkgs = import nixpkgs {
+      inherit system;
+      overlays = [ self.overlays.default ];
+      config.allowBroken = true;
+    };
+  in {
+    devShells.default = pkgs.haskellPackages.shellFor {
+      packages = p: [ p.gtk-strut ];
+      nativeBuildInputs = with pkgs.haskellPackages; [
+        cabal-install hlint ghcid ormolu implicit-hie haskell-language-server
+      ];
+    };
+    packages.default = pkgs.haskellPackages.gtk-strut;
+  }) // {
+    overlays.default = final: prev: {
       haskellPackages = prev.haskellPackages.override (old: {
         overrides = prev.lib.composeExtensions (old.overrides or (_: _: {}))
         (hself: hsuper: {
@@ -19,17 +33,5 @@
         });
       });
     };
-    overlays = [ overlay ];
-  in flake-utils.lib.eachDefaultSystem (system:
-  let pkgs = import nixpkgs { inherit system overlays; config.allowBroken = true; };
-  in
-  rec {
-    devShell = pkgs.haskellPackages.shellFor {
-      packages = p: [ p.gtk-strut ];
-      nativeBuildInputs = with pkgs.haskellPackages; [
-        cabal-install hlint ghcid ormolu implicit-hie haskell-language-server
-      ];
-    };
-    defaultPackage = pkgs.haskellPackages.gtk-strut;
-  }) // { inherit overlay overlays; } ;
+  };
 }