Fix picking CI builds

The conflict happens when we try to update a build,
but fail to do so due to fact that we update the same build concurrently for two different runners.

app/services/ci/register_build_service.rb

@@ -29,10 +29,10 @@ module Ci
         build.run!
       end
 
-      build
+      [build, true]
 
     rescue StateMachines::InvalidTransition, ActiveRecord::StaleObjectError
-      nil
+      [nil, false]
     end
 
     private

lib/ci/api/builds.rb

@@ -23,8 +23,9 @@ module Ci
 
           new_update = current_runner.ensure_runner_queue_value
 
-          build = Ci::RegisterBuildService.new(current_runner).execute
+          build, valid = Ci::RegisterBuildService.new(current_runner).execute
 
+          if valid
             if build
               Gitlab::Metrics.add_event(:build_found,
                                         project: build.project.path_with_namespace)
@@ -37,6 +38,11 @@ module Ci
 
               build_not_found!
             end
+          else
+            # We received build that is invalid due to concurrency conflict
+            Gitlab::Metrics.add_event(:build_invalid)
+            conflict!
+          end
         end
 
         # Update an existing build - Runners only