Unclear on which computer (master, slave, or Maven) the `getDigestOf` call is occurring. For efficiency it had better be on master, since that is where the `InputStream` is being served.

Anyway this logic is quite dubious. Better to unconditionally send the artifact, and to not try to send the same artifact twice. A broader issue is that Fast Archiver (and probably other managers, such as a ZIP-based one) cannot work efficiently when given files one by one; they prefer to work in batch on the whole workspace. But the Maven plugin does not archive artifacts using workspace-relative paths, meaning that the existing `archive` method could not be used even if we batched them all up to the end. So the manager signature probably needs to change, to accept a `Map<String,FilePath>`.