packer/plugin: remove race in Exited()
packer/plugin: remove race in Exited()

patricklucas/packer,TranscendComputing/packer,Kaixiang/packer,Kaixiang/packer,suma/packer,patricklucas/packer,suma/packer,Kaixiang/packer,TranscendComputing/packer,TranscendComputing/packer,patricklucas/packer,Kaixiang/packer,suma/packer,suma/packer,patricklucas/packer,TranscendComputing/packer