nixos/test-driver: use function instead of var

Use a proper function to enable serial log printing instead of setting
class member variables directly.

nixos/lib/test-driver/test_driver/driver.py

@@ -249,10 +249,10 @@ class Driver:
         )
 
     def serial_stdout_on(self) -> None:
-        rootlog._print_serial_logs = True
+        rootlog.print_serial_logs(True)
 
     def serial_stdout_off(self) -> None:
-        rootlog._print_serial_logs = False
+        rootlog.print_serial_logs(False)
 
     def check_polling_conditions(self) -> None:
         for condition in self.polling_conditions: