Use overlay

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

flake.nix

@@ -22,7 +22,14 @@
         treefmt = inputs.treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
       in
       {
-        lib.${system} = import ./lib { inherit pkgs; };
+        overlays =
+          let
+            lib = import ./overlay;
+          in
+          {
+            inherit lib;
+            default = lib;
+          };
 
         formatter.${system} = treefmt.config.build.wrapper;
         checks.${system}.formatting = treefmt.config.build.check inputs.self;