arrow

ARROW-11941: [Dev] Don't update Jira if run "DEBUG=1 merge_arrow_pr.py"

When environment variable DEBUG=1, merge_arrow_pr.py should only try
the merge steps without updating anything. Though PR status is not
changed, Jira issue is updated even if DEBUG=1. This patch fixes it.

Closes #9706 from cyb70289/merge-pr-debug

Authored-by: Yibo Cai <yibo.cai@arm.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>