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.

src/check.c

@@ -1391,7 +1391,7 @@ int start_check_task(struct check *check, int mininter,
 
 	/* task for the check. Process-based checks exclusively run on thread 1. */
 	if (check->type == PR_O2_EXT_CHK)
-		t = task_new_on(1);
+		t = task_new_on(0);
 	else
 		t = task_new_anywhere();