nixos: Make system.build.vm a standard attribute based on vmVariant

flake.nix

···
                   system.nixos.versionSuffix =
                     ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
                   system.nixos.revision = final.mkIf (self ? rev) self.rev;
-
-                  system.build = {
-                    vm = lib.mkDefault config.virtualisation.vmVariant.system.build.vm;
-                    vmWithBootLoader = lib.mkDefault config.virtualisation.vmVariantWithBootLoader.system.build.vm;
-                  };
                 })
               ];
           });

nixos/default.nix

···
 
   system = eval.config.system.build.toplevel;
 
-  vm = eval.config.virtualisation.vmVariant.system.build.vm;
-
-  vmWithBootLoader = eval.config.virtualisation.vmVariantWithBootLoader.system.build.vm;
+  inherit (eval.config.system.build) vm vmWithBootLoader;
 }

nixos/modules/virtualisation/build-vm.nix

···
-{ extendModules, lib, ... }:
+{ config, extendModules, lib, ... }:
 let
 
   inherit (lib)
···
       inherit (vmVariantWithBootLoader) type;
       default = {};
       visible = "shallow";
+    };
+
+  };
+
+  config = {
+
+    system.build = {
+      vm = lib.mkDefault config.virtualisation.vmVariant.system.build.vm;
+      vmWithBootLoader = lib.mkDefault config.virtualisation.vmVariantWithBootLoader.system.build.vm;
     };
 
   };