Use new packages.${system}.default convention

The switch from defaultPackage to packages.${system}.default happened in NixOS/nix#5532.

Author: thkoch2001

flake-template/flake.nix

@@ -8,7 +8,7 @@
     let
       system = "x86_64-linux";
     in {
-      defaultPackage.${system} = self.packages.${system}.my-microvm;
+      packages.${system}.default = self.packages.${system}.my-microvm;
 
       packages.${system} = {
         my-microvm = self.nixosConfigurations.my-microvm.config.microvm.declaredRunner;