substitute: Fix file descriptor leak in 'http-multiple-get'.

In practice we would not leak much since we reconnect after ~100
requests (with nginx running on hydra.gnu.org.)

* guix/scripts/substitute.scm (http-multiple-get): Call 'close-port'
  before 'connect'.

guix/scripts/substitute.scm

@@ -467,6 +467,7 @@ to read the response body.  Return the list of results."
              ;; case we have to try again.  Check whether that is the case.
              (match (assq 'connection (response-headers resp))
                (('connection 'close)
+                (close-port p)
                 (connect requests result))        ;try again
                (_
                 (loop tail                        ;keep going