style(flake.nix): make it one line

Author: ryoppippi

flake.nix

@@ -12,9 +12,7 @@
 
     rust-overlay = {
       url = "github:oxalica/rust-overlay";
-      inputs = {
-        nixpkgs.follows = "nixpkgs";
-      };
+      inputs.nixpkgs.follows = "nixpkgs";
     };
   };