When we attempt to determine whether a remote spec (in a binary mirror)
is up-to-date or needs to be rebuilt, we compare the full_hash stored in
the remote spec.yaml file against the full_hash computed from the local
concrete spec. Since the full_hash moved into the spec (and is no longer
at the top level of the spec.yaml), we need to look there for it. This
oversight from #18359 was causing all specs to get rebuilt when the
full_hash wasn't fouhd at the expected location.
7cfdf61979