Make setup and teardown less noisy for snap builds

[Evan]

From https://github.com/canonical-websites/build.snapcraft.io/issues/834:

We display a few lines of tear down (and setup) around the snapcraft run, but I can't think of a situation under which these would be useful to the developer debugging their failed build (but do correct me if you can think of one).

These can be a distraction from the actual source of the failure, which my brain assumes will be on the very bottom of the log.

[Colin Watson (cjwatson)]

This is less noisy than it was following https://code.launchpad.net/~cjwatson/launchpad-buildd/faster-cleanup/+merge/344938, but it's probably still possible to do better.