substitute: Download nar from another server upon ETIMEDOUT.

Previously, 'guix substitute' would fail abruptly with something like:

  guix substitute: warning: while fetching https://ci.guix.gnu.org/nar/lzip/…-example: server is somewhat slow
  guix substitute: warning: try `--no-substitutes' if the problem persists
  guix substitute: error: connect*: Connection timed out
  substitution of /gnu/store/…-example failed

* guix/scripts/substitute.scm (network-error?): Add ETIMEDOUT.

guix/scripts/substitute.scm

@@ -577,7 +577,7 @@ STATUS-PORT."
       (or (and (system-error? exception)
                (let ((errno (system-error-errno
                              (cons 'system-error (exception-args exception)))))
-                 (memv errno (list ECONNRESET ECONNABORTED
+                 (memv errno (list ECONNRESET ECONNABORTED ETIMEDOUT
                                    ECONNREFUSED EHOSTUNREACH
                                    ENOENT))))     ;for "file://"
           (and (kind-and-args? exception)