[buildmaster] Only rename downloaded files from builder after all downloads succeed

[waddlesplash]

Basically, move the rename out of this loop: https://github.com/haikuports/haikuporter/blob/master/HaikuPorter/Builders/RemoteBuilderSSH.py#L339

With the current model, if one file succeeds in downloading but the others do not, then the build will ultimately fail, but we will get "error some packages already exists" when retrying it, and those stray packages have to be manually deleted.