shutdown: unload perilog plugin during shutdown

Problem: when flux is shutting down and canceling running
jobs, the epilog can greatly extend the shutdown time.

Unload the perilog plugin as part of the cleanup tasks so that
canceled jobs can complete quickly in the shutdown case.

etc/rc1

@@ -113,6 +113,7 @@ fi
 if test $RANK -eq 0; then
     if test -z "${FLUX_DISABLE_JOB_CLEANUP}"; then
 	flux admin cleanup-push <<-EOT
+	flux jobtap remove perilog 2>/dev/null || true
 	flux queue stop --quiet --all --nocheckpoint
 	flux cancel --user=all --quiet --states RUN
 	flux queue idle --quiet