Channels can now advertise a binary cache by creating a file
<channel-url>/binary-cache-url.  The channel unpacker puts these in
its "binary-caches" subdirectory.  Thus, the URLS of the binary caches
for the channels added by root appear in
/nix/var/nix/profiles/per-user/eelco/channels/binary-caches/*.  The
binary cache substituter reads these and adds them to the list of
binary caches.