Copy the port monitoring fiber from the build coordinator

As the data service has the same issue where it stops listening on the port.

guix-data-service/web/server.scm

@@ -144,6 +144,11 @@ port. Also, the port used can be changed by passing the --port option.\n"
                    #:host host
                    #:port port))
                 #:unwind? #t)))
+
+          ;; Guile sometimes just seems to stop listening on the port, so try
+          ;; and detect this and quit
+          (spawn-port-monitoring-fiber port finished?)
+
           (wait finished?))
         #:parallelism 4))
      finished?)))