Labeled Durable Tasks Reported as Stage

DurableTaskSteps now support labels, but [using a label](https://github.com/jenkinsci/workflow-durable-task-step-plugin/blob/master/src/main/java/org/jenkinsci/plugins/workflow/steps/durable_task/DurableTaskStep.java#L145-L147) results in a call to `FlowNode.addAction` causing the plugin to report the step as a stage.

The plugin appears to recognize these steps are not really stages, as the status is set to pending early on but not marked successful/failed until the end of execution which leads me to believe it's getting resolved by the close method.

Because stages can be identified ahead of time for declarative pipelines unlike scripted pipelines, I presume they are unaffected.