* guix/scripts/substitute-binary.scm (%narinfo-cache-directory,
%narinfo-ttl, %narinfo-negative-ttl): New variables.
(with-atomic-file-output, object->fields, read-narinfo,
write-narinfo, narinfo->string, string->narinfo, lookup-narinfo): New
procedures.
(fetch-narinfo): Adjust to use `read-narinfo'.
(guix-substitute-binary): Ensure the existence of
%NARINFO-CACHE-DIRECTORY. Use `lookup-narinfo' instead of
`fetch-narinfo'.
eba783b7b2