It's already async, from Project#execute_services, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6342/diffs#note_16132555

app/models/project_services/pipelines_email_service.rb

@@ -32,7 +32,7 @@ class PipelinesEmailService < Service
     return unless all_recipients.any?
 
     pipeline_id = data[:object_attributes][:id]
-    SendPipelineNotificationWorker.perform_async(pipeline_id, all_recipients)
+    SendPipelineNotificationWorker.new.perform(pipeline_id, all_recipients)
   end
 
   def can_test?