DEV: only use the records that are auto populated by the task. (#14360)

Previously, it was using existing user and topic records to generate random posts.

lib/discourse_dev/user.rb

@@ -44,7 +44,7 @@ module DiscourseDev
     end
 
     def self.random
-      super(::User)
+      super(::User, use_existing_records: false)
     end
 
     def set_random_avatar(user)