substitute-binary: Store the cache's URI in the local cached narinfo.

* guix/scripts/substitute-binary.scm (<narinfo>)[uri-base]: New field.
  (narinfo-maker): Pass CACHE-URL as the 'uri-base' value.
  (string->narinfo): Add 'cache-uri' parameter.
  (lookup-narinfo)[cache-entry]: Switch to version 1.  Add 'cache-uri'
  field.  Adjust body accordingly.
  (remove-expired-cached-narinfos): Switch to version 1 by default.