shutdown: skip epilog on jobs canceled at shutdown

Problem: the epilog can sometimes take a long time, which
delays shutdown when there are running jobs.

Unload the perilog jobtap plugin before canceling jobs in the
shutdown script.

etc/shutdown

@@ -7,6 +7,9 @@ while getopts f namer; do
     esac
 done
 
+# Avoid epilog on jobs canceled after this point
+flux jobtap remove perilog 2>/dev/null
+
 flux queue stop --quiet --all --nocheckpoint
 flux cancel --user=all --quiet --states RUN