refactor(flake): move custom packages to overlay

src/flake/default.nix

@@ -5,7 +5,18 @@
   root,
   system,
 }:
+let
+  overlay = import ./overlay.nix { inherit root; };
+  pkgs = import inputs.nixpkgs {
+    inherit system;
+    overlays = [ overlay ];
+  };
+in
 {
+  overlays.default = overlay;
+
+  packages.${system} = pkgs.alexion;
+
   nixosConfigurations = import ../dotfiles {
     inherit
       inputs