slaprunner: Export (rsync) silence rsync warning

When a file is removed while rsync is running (after build the list and before transfer the file), rsync fail with the follow message:

 rsync warning: some files vanished before they could be transferred

Long back rafael proped a solution here 387d3b7e But it seems like adding --ignore-missing-args dont really help but just ignore the files given in args and are counldn't be found anymore.