The problem here is that newly introduced behavior in commit
b2b0409be462aaa90de99791a57d4c7a6ce12e6d resulted in builddir and
destdir *always* being removed when starting a build, as a part of
masterdir auto-update. We don't want this as we may want to resume
a previous build (e.g. by running stages individually or by resuming
a failed build from where it stopped).
Therefore, explicitly override the removal to restore previous behavior.
4098c62ebc