define default flake, pkg overlay

flake.nix

@@ -31,7 +31,7 @@
           pkgs = nixpkgsFor.${system};
         in
         {
-          weather = pkgs.buildGoModule {
+          default = pkgs.buildGoModule {
             pname = "weather";
             inherit version;
             # In 'nix develop', we don't need a copy of the source tree
@@ -52,9 +52,8 @@
           };
         });
 
-      # The default package for 'nix build'. This makes sense if the
-      # flake provides only one package or there is a clear "main"
-      # package.
-      defaultPackage = forAllSystems (system: self.packages.${system}.weather);
+       overlays.default = (final: prev: {
+          weather = self.packages.${prev.system}.default;
+       });
     };
 }