substitute: Make room for a 'ttl' field in cached entries.

* guix/scripts/substitute.scm (cached-narinfo): Expect 'narinfo' sexp
version 2 with a 'ttl' field.
(cache-narinfo!)[cache-entry]: Produce 'narinfo' sexp version 2 with a
'ttl' field.
(remove-expired-cached-narinfos)[expired?]: Read 'narinfo' sexp version 2.