Order leased jobs by serial (#2912)

This will ensure the job leased first, gets send to the cluster first

Currently we just order by postgres default sorting - which often picks the most recently leased - causing the first lease jobs to get stuck
 - This only occurs when scheduling is faster than leasing

internal/scheduler/database/job_repository.go

@@ -244,6 +244,7 @@ func (r *PostgresJobRepository) FetchJobRunLeases(ctx context.Context, executor
 				AND jr.succeeded = false
 				AND jr.failed = false
 				AND jr.cancelled = false
+				ORDER BY jr.serial
 				LIMIT %d;
 `