postgresql-init-db: Fix installation from world-unreadable directory.

This reverts part of commit 476524c0c1
(#18215), to fix this error when running the installer from a
directory that isn’t world-readable:

+ '[' -e /var/run/supervisor.sock ']'
+++ dirname /root/zulip-server-4.1/scripts/setup/postgresql-init-db
++ dirname /root/zulip-server-4.1/scripts/setup
+ su zulip -c /root/zulip-server-4.1/scripts/stop-server
bash: /root/zulip-server-4.1/scripts/stop-server: Permission denied

Zulip installation failed (exit code 126)!

Signed-off-by: Anders Kaseorg <anders@zulip.com>

scripts/setup/postgresql-init-db

@@ -28,7 +28,7 @@ fi
 
 # Shut down all services to ensure a quiescent state.
 if [ -e "/var/run/supervisor.sock" ]; then
-    su zulip -c "$(dirname "$(dirname "$0")")/stop-server"
+    supervisorctl stop all
 fi
 
 # Drop any open connections to any old database.