nix: move overlays section from flake-utils

flake.nix

@@ -133,6 +133,13 @@
   };
 
   outputs = inputs @ { self, nixpkgs, utils, ... }:
+    {
+      overlays = {
+        default = final: prev: {
+          myneovim = prev.callPackage ./. { };
+        };
+      };
+    } //
     utils.lib.eachDefaultSystem (system:
       let
         inputPlugins = [
@@ -206,9 +213,5 @@
             pkgs.stylua # lua formatter
           ];
         };
-
-        overlays = f: p: {
-          myneovim = neovim;
-        };
       });
 }