agent discover cron job - email every minute

There is a cron job installed as `frappe` which performs agent discovery every minute:

`* * * * * cd /home/frappe/agent && /home/frappe/agent/env/bin/agent discover`

This results in an email being sent to the frappe user every minute:

```
From frappe@ip-10-1-2-101.eu-west-2.compute.internal Sat Aug 24 18:44:03 2024
X-Original-To: frappe
From: root@ip-10-1-2-101.eu-west-2.compute.internal (Cron Daemon)
To: frappe@ip-10-1-2-101.eu-west-2.compute.internal
Subject: Cron cd /home/frappe/agent && /home/frappe/agent/env/bin/agent discover
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
X-Cron-Env:
X-Cron-Env:
X-Cron-Env:
X-Cron-Env:
Date: Sat, 24 Aug 2024 18:44:03 +0000 (UTC)

rendering /tmp/agent-prometheus-cluster-g8x_86zx.yml
rendering /tmp/agent-prometheus-tls-e00f0smc.yml
rendering /tmp/agent-prometheus-sites-luugtbe1.yml
rendering /tmp/agent-prometheus-domains-v4umh3vr.yml
```

Fix by redirecting stdout to /dev/null:

`* * * * * cd /home/frappe/agent && /home/frappe/agent/env/bin/agent discover > /dev/null`