substitute: Do not discard ttl info when the response code is 202.

* guix/scripts/substitute.scm (fetch-narinfos)
[handle-narinfo-response]: Add code 202 to the condition.  This will
allow in the future the use of 404 for nar requests which cannot be
built in the present, leaving for the 202 to indicate that a request has
triggered a nar generation that might be completed later, as specified
in <https://www.ietf.org/rfc/rfc2616.txt>.

guix/scripts/substitute.scm

@@ -656,7 +656,7 @@ print a warning and return #f."
                 (get-bytevector-n port len)
                 (read-to-eof port))
             (cache-narinfo! url (hash-part->path hash-part) #f
-                            (if (= 404 code)
+                            (if (or (= 404 code) (= 202 code))
                                 ttl
                                 %narinfo-transient-error-ttl))
             result))))