installer: connman: Fix Wifi connection.

Issue reported by SergioBG here:
https://lists.gnu.org/archive/html/bug-guix/2020-04/msg00432.html.

Due to default buffering mode, the installer was reading from the pipe without
having actually written a command before.

* gnu/installer/connman.scm (connman-connect-with-auth): Set pipe buffering
mode to 'line.

gnu/installer/connman.scm

@@ -393,6 +393,7 @@ interactive connmanctl process."
     (dynamic-wind
       (const #t)
       (lambda ()
+        (setvbuf pipe 'line)
         (run-connection-sequence pipe)
         #t)
       (lambda ()