Add nix module to preserve system flake in generations

flake.nix

@@ -16,9 +16,10 @@
         specialArgs = { inherit system inputs; };
 
         modules = [
-          ./systems/lenovo.nix
+          ./modules/preserve-system.nix
           ./modules/core.nix
           ./modules/user
+          ./systems/lenovo.nix
         ];
       };
     };

modules/preserve-system.nix

@@ -0,0 +1,7 @@
+# preserve the flake that built the current system generation
+
+{ inputs, ... }: let
+  inherit (inputs) self;
+in {
+  environment.etc."current-flake".source = self;
+}