Mark the ExpirePipelineCacheWorker as idempotent

This worker expires cache, so it is already idempotent: expiring the
cache twice does nothing.

app/workers/all_queues.yml

@@ -709,7 +709,7 @@
   :urgency: :high
   :resource_boundary: :cpu
   :weight: 3
-  :idempotent: 
+  :idempotent: true
 - :name: pipeline_creation:create_pipeline
   :feature_category: :continuous_integration
   :has_external_dependencies: 

app/workers/expire_pipeline_cache_worker.rb

 # frozen_string_literal: true
 
-class ExpirePipelineCacheWorker # rubocop:disable Scalability/IdempotentWorker
+class ExpirePipelineCacheWorker
   include ApplicationWorker
   include PipelineQueue
 
@@ -8,6 +8,8 @@ class ExpirePipelineCacheWorker # rubocop:disable Scalability/IdempotentWorker
   urgency :high
   worker_resource_boundary :cpu
 
+  idempotent!
+
   # rubocop: disable CodeReuse/ActiveRecord
   def perform(pipeline_id)
     pipeline = Ci::Pipeline.find_by(id: pipeline_id)

spec/workers/expire_pipeline_cache_worker_spec.rb

@@ -11,7 +11,9 @@ describe ExpirePipelineCacheWorker do
 
   describe '#perform' do
     it 'executes the service' do
-      expect_any_instance_of(Ci::ExpirePipelineCacheService).to receive(:execute).with(pipeline).and_call_original
+      expect_next_instance_of(Ci::ExpirePipelineCacheService) do |instance|
+        expect(instance).to receive(:execute).with(pipeline).and_call_original
+      end
 
       subject.perform(pipeline.id)
     end
@@ -31,5 +33,9 @@ describe ExpirePipelineCacheWorker do
 
       subject.perform(pipeline.id)
     end
+
+    it_behaves_like 'an idempotent worker' do
+      let(:job_args) { [pipeline.id] }
+    end
   end
 end