Syncing from upstream OCA/queue (12.0) - #577

…pone

When a job's perform() raised a PG serialization error, the runjob controller wrapped it as RetryableJobError and called retry_postpone(), which reassigns job.env to a temporary cursor inside a `with` block and closes it on exit. The handler then fell through to _enqueue_dependent_jobs() -> Job.enqueue_waiting(), running self.env.cr.execute() on that now-closed cursor and raising
"psycopg2.OperationalError: Unable to use a closed cursor".

A postponed job is not done and must not release its dependent jobs, so return right after the rollback. This also avoids touching the closed cursor. Aligns 12.0 with the upstream 14.0+ behaviour.