Fix exception in legacy /post/index.xml endpoint.

Fixes intermittent `uninitialized constant LegacyController::Builder` exceptions.

Gemfile

@@ -44,6 +44,7 @@ gem 'webpacker', '>= 4.0.x'
 gem 'retriable'
 gem 'redis'
 gem 'request_store'
+gem 'builder'
 gem 'term-ansicolor', require: "term/ansicolor"
 
 # needed for looser jpeg header compat

Gemfile.lock

@@ -451,6 +451,7 @@ DEPENDENCIES
   bcrypt
   bigquery!
   bootsnap
+  builder
   capistrano (~> 3.10)
   capistrano-deploytags (~> 1.0.0)
   capistrano-rails

app/controllers/legacy_controller.rb

@@ -7,7 +7,7 @@ class LegacyController < ApplicationController
 
     respond_with(@posts) do |format|
       format.xml do
-        xml = Builder::XmlMarkup.new(indent: 2)
+        xml = ::Builder::XmlMarkup.new(indent: 2)
         xml.instruct!
         xml.posts do
           @posts.each { |attrs| xml.post(attrs) }