Refine build entity tests a little

spec/serializers/build_entity_spec.rb

@@ -1,10 +1,12 @@
 require 'spec_helper'
 
 describe BuildEntity do
-  subject do
-    described_class.new(build, request: double).as_json
+  let(:entity) do
+    described_class.new(build, request: double)
   end
 
+  subject { entity.as_json }
+
   context 'when build is a regular job' do
     let(:build) { create(:ci_build) }