Throwaway diagnostic, not for merge. Builds the Arch leg's real command
(--server --debug --test) three times against a 6G bound and a cache the job
owns, so the working set can be read off du and reproducibility tested without
touching the repository's cache quota. Also recompresses the cache to measure
what the same configuration costs compressed, since a compressed cache is the
only way one configuration can fit a per-leg bound.
