It also looks like we never use

workspace.materialize_directories()

. We only use the singular version; every use case we have calls

MergeDigests

beforehand to get a single

Digest

. Afaict, the only time where this would be really useful is that you can have different `path_prefix`s per each digest. But now that you must write to the buildroot—and that we have `AddPrefix`—this doesn’t seem that important. Fine to remove?