Use "nix cat-store" to serve files from the Nix store

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.

Created by  Eelco Dolstra  on October 18, 2017
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