refactor(flake): define package overlay inline

src/flake/default.nix

@@ -5,22 +5,19 @@
   root,
   system,
 }:
-let
-  overlay = final: _prev: {
+{
+  overlays.default = final: _prev: {
     alexion = import ./packages {
       pkgs = final;
       inherit root;
     };
   };
-  pkgs = import inputs.nixpkgs {
-    inherit system;
-    overlays = [ overlay ];
-  };
-in
-{
-  overlays.default = overlay;
 
-  packages.${system} = pkgs.alexion;
+  packages.${system} =
+    (import inputs.nixpkgs {
+      inherit system;
+      overlays = [ self.overlays.default ];
+    }).alexion;
 
   nixosConfigurations = import ../dotfiles {
     inherit