Update flake to use default nixpkgs source for treefmt-nix

Author: benalleng

flake.lock

@@ -9,7 +9,10 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
     crane.url = "github:ipetkov/crane";
-    treefmt-nix.url = "github:numtide/treefmt-nix";
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs =