Fix rubocop offense in build stage migration classes

lib/gitlab/background_migration/migrate_build_stage.rb

@@ -40,7 +40,7 @@ module Gitlab
         # In that case we just don't migrate such stage.
         #
         def legacy?
-         @build.stage.nil?
+          @build.stage.nil?
         end
 
         def ensure!

spec/migrations/schedule_build_stage_migration_spec.rb

@@ -8,6 +8,8 @@ describe ScheduleBuildStageMigration, :migration do
   let(:jobs) { table(:ci_builds) }
 
   before do
+    stub_const("#{described_class}::BATCH", 1)
+
     ##
     # Dependencies
     #
@@ -24,10 +26,6 @@ describe ScheduleBuildStageMigration, :migration do
     jobs.create!(id: 4109, commit_id: 1, project_id: 123, stage_id: 1)
   end
 
-  before do
-    stub_const("#{described_class}::BATCH", 1)
-  end
-
   it 'schedules delayed background migrations in batches in bulk' do
     Sidekiq::Testing.fake! do
       Timecop.freeze do