BUG/MEDIUM: checks: fix the starting thread for external checks

BUG/MEDIUM: checks: fix the starting thread for external checks

When cleaning up the code to remove most explicit task masks in commit
beeabf531 ("MINOR: task: provide 3 task_new_* wrappers to simplify the
API"), a mistake was done with the external checks where the call does
task_new_on(1) instead of task_new_on(0) due to the confusion with the
previous mask 1.

No backport is needed as that's only 2.5-dev.