kernel: Buffer the output and error ports.

* guix-jupyter-kernel.scm <top level>: Add 'setvbuf' calls.

guix-jupyter-kernel.scm

@@ -735,6 +735,9 @@ environment ~s (PID ~s)~%"
   (sigaction SIGTERM (exit-handler kernel))
   (sigaction SIGINT  (exit-handler kernel))
 
+  (setvbuf (current-output-port) 'line)
+  (setvbuf (current-error-port) 'line)
+
   (format/log "Guix kernel started (PID ~a)~%" (getpid))
   (with-store store
     ;; Enable "build traces" so we can use (guix status) to track build and