publish: Increase TCP buffer size when sending non-cached nars.

This increases throughput tenfold (!) on localhost when downloading
large nars without '--cache'.  This is a followup to
5e3d169945.

Reported by Maxim Cournoyer.

* guix/scripts/publish.scm (http-write): In the
'application/x-nix-archive' case, add call to 'setsockopt'.

guix/scripts/publish.scm

@@ -891,6 +891,8 @@ blocking."
                                             client))
                (port        (begin
                               (force-output client)
+                              (setsockopt client SOL_SOCKET
+                                          SO_SNDBUF (* 128 1024))
                               (nar-response-port response compression))))
           ;; XXX: Given our ugly workaround for <http://bugs.gnu.org/21093> in
           ;; 'render-nar', BODY here is just the file name of the store item.