Copy scheduled_jobs list before sorting it

The start_scheduled_jobs function mistakenly sorts the scheduled_jobs
list in-place. As a result, when the ts_update_scheduled_jobs_list
function compares the updated list of scheduled jobs with the existing
scheduled jobs list, it is comparing a list that is sorted by job_id to
one that is sorted by next_start time. Fix that by properly copying the
scheduled_jobs list into a new list and use that for sorting.

Fixes timescale#5537

src/bgw/scheduler.c

@@ -575,7 +575,7 @@ start_scheduled_jobs(register_background_worker_callback_type bgw_register)
 	ordered_scheduled_jobs = list_qsort(scheduled_jobs, cmp_next_start);
 #else
 	/* PG13 does in-place sort */
-	ordered_scheduled_jobs = scheduled_jobs;
+	ordered_scheduled_jobs = list_copy(scheduled_jobs);
 	list_sort(ordered_scheduled_jobs, cmp_next_start);
 #endif
 
@@ -588,9 +588,7 @@ start_scheduled_jobs(register_background_worker_callback_type bgw_register)
 			scheduled_ts_bgw_job_start(sjob, bgw_register);
 	}
 
-#if PG13_LT
 	list_free(ordered_scheduled_jobs);
-#endif
 }
 
 /* Returns the earliest time the scheduler should start a job that is waiting to be started */