When the workflow is triggered by removing the 'conflicts' label
(pull_request_target unlabeled event), github.event.issue.number is
not available. Use github.event.pull_request.number as fallback.
Fixes: https://scylladb.atlassian.net/browse/RELENG-245
Closes scylladb/scylladb#28543
(cherry picked from commit b30ecb72d5)
Closes scylladb/scylladb#28553