packer/plugin: confirm cleanup at first signal received

Although I "knew" packer was getting my ctrl-c's, I figured for people less intimately familiar with its behavior that it'd be good to confirm that the signal was received, and that a cleanup is occurring, or will occur. I found in certain steps that the UI feedback from the builder-implemented cleanup would occur after that previous step finished running – this would lead to some time of no UI feedback, even after a ctrl-c was sent.