Fix /actor needlessly reading session cookie and varying on Signature (#24664)

app/controllers/instance_actors_controller.rb

 # frozen_string_literal: true
 
-class InstanceActorsController < ApplicationController
-  include AccountControllerConcern
+class InstanceActorsController < ActivityPub::BaseController
+  vary_by ''
 
-  skip_before_action :check_account_confirmation
-  skip_around_action :set_locale
+  serialization_scope nil
+
+  before_action :set_account
+  skip_before_action :require_functional!
+  skip_before_action :update_user_sign_in
 
   def show
     expires_in 10.minutes, public: true