Just draw :legacy_builds

config/routes/legacy_builds.rb

+resources :builds, only: [:index, :show], constraints: { id: /\d+/ } do
+  collection do
+    resources :artifacts, only: [], controller: 'build_artifacts' do
+      collection do
+        get :latest_succeeded,
+          path: '*ref_name_and_path',
+          format: false
+      end
+    end
+  end
+
+  member do
+    get :raw
+  end
+
+  resource :artifacts, only: [], controller: 'build_artifacts' do
+    get :download
+    get :browse, path: 'browse(/*path)', format: false
+    get :file, path: 'file/*path', format: false
+    get :raw, path: 'raw/*path', format: false
+  end
+end

config/routes/project.rb

 require 'constraints/project_url_constrainer'
-require 'gitlab/routes/legacy_builds'
 
 resources :projects, only: [:index, :new, :create]
 
@@ -253,7 +252,7 @@ constraints(ProjectUrlConstrainer.new) do
         end
       end
 
-      Gitlab::Routes::LegacyBuilds.new(self).draw
+      draw :legacy_builds
 
       resources :hooks, only: [:index, :create, :edit, :update, :destroy], constraints: { id: /\d+/ } do
         member do

lib/gitlab/routes/legacy_builds.rb

-module Gitlab
-  module Routes
-    class LegacyBuilds
-      def initialize(map)
-        @map = map
-      end
-
-      def draw
-        @map.instance_eval do
-          resources :builds, only: [:index, :show], constraints: { id: /\d+/ } do
-            collection do
-              resources :artifacts, only: [], controller: 'build_artifacts' do
-                collection do
-                  get :latest_succeeded,
-                    path: '*ref_name_and_path',
-                    format: false
-                end
-              end
-            end
-
-            member do
-              get :raw
-            end
-
-            resource :artifacts, only: [], controller: 'build_artifacts' do
-              get :download
-              get :browse, path: 'browse(/*path)', format: false
-              get :file, path: 'file/*path', format: false
-              get :raw, path: 'raw/*path', format: false
-            end
-          end
-        end
-      end
-    end
-  end
-end