etcupdate: Don't rotate trees for a dry run.

When performing a dry run, remove the temporary tree created rather
than rotating the trees. Rotating the trees meant that etcupdate
thought the latest changes were already merged and would not merge
them on the next real run.

PR: 260281
Reported by: Tomoaki AOKI <junchoon@dec.sakura.ne.jp>
Fixes: 0611aec3cf3a etcupdate: Always extract to a temporary tree.
MFC after: 1 week