services: guix-build-coordinator: Configure output buffering.

Otherwise the logging is garbled.

* gnu/services/guix.scm (make-guix-build-coordinator-start-script): Configure
line output buffering for stdout and stderr.

gnu/services/guix.scm

@@ -200,6 +200,9 @@
                       (guix-build-coordinator build-allocator)
                       (guix-build-coordinator coordinator))
 
+         (setvbuf (current-output-port) 'line)
+         (setvbuf (current-error-port) 'line)
+
          (simple-format #t "starting the guix-build-coordinator:\n  ~A\n"
                         (current-filename))
          (let* ((metrics-registry (make-metrics-registry