This makes downloading/viewing build results work with binary cache stores. For good performance, this should be used in conjunction with https://github.com/NixOS/nix/commit/ca580bec35ea4d1984e36864158d7be99cfcb34b, i.e. you should set store_uri to something like s3://my-cache?local-nar-cache=/tmp/nar-cache to cache NARs between requests.