Use overlay

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

flake.nix

@@ -10,20 +10,22 @@
 
   outputs =
     inputs:
-    (
+    {
+      overlays.default = import ./overlays;
+    }
+    // (
       let
         system = "x86_64-linux";
 
         pkgs = import inputs.nixpkgs {
           inherit system;
           config.allowUnfree = true;
+          overlays = [ inputs.self.overlays.default ];
         };
 
         treefmt = inputs.treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
       in
       {
-        lib.${system} = import ./lib { inherit pkgs; };
-
         formatter.${system} = treefmt.config.build.wrapper;
         checks.${system}.formatting = treefmt.config.build.check inputs.self;
       }