Use more direct attribute handling in `User` fabricator (#30495)

spec/fabricators/user_fabricator.rb

@@ -1,7 +1,12 @@
 # frozen_string_literal: true
 
 Fabricator(:user) do
-  account      { Fabricate.build(:account, user: nil) }
+  account do |attrs|
+    Fabricate.build(
+      :account,
+      attrs.fetch(:account_attributes, {}).merge(user: nil)
+    )
+  end
   email        { sequence(:email) { |i| "#{i}#{Faker::Internet.email}" } }
   password     '123456789'
   confirmed_at { Time.zone.now }