Datanode plugin path

In your download file graylog-datanode-6.3.9-linux-x64.tgz the plugin path is absolute and point to the filesystem root (/plugin).

## Expected Behavior

The plugin path should be relative to the installed path (Ex: DATANODE_ROOT/plugin).
`2026-02-25T00:46:45.629420+00:00 an-00003 graylog-datanode[3521787]: INFO [CmdLineTool] Loaded plugin: Trino Node 6.3.9 [org.graylog.trino.TrinoPlugin]`

## Current Behavior

The plugin path doesn't exist. The plugin trino-node is not loaded.
`2026-02-25T00:38:47.911231+00:00 an-00003 graylog-datanode[3510914]: WARN [PluginLoader] Plugin directory /plugin does not exist, not loading plugins.`

## Possible Solution

Temporary solution: ln -s /opt/graylog-datanode/plugin /plugin

## Steps to Reproduce (for bugs)

1. Install in /opt/graylog-datanode
2. Create a service file graylog-datanode.service
3. Start the service

## Context

Trying to get graylog-datanode to start correctly.

## Your Environment

* Graylog Version: 6.3.9
* Java Version: Bundled
* OpenSearch Version: 1.3.20
* MongoDB Version: 7.0.28
* Operating System: Amazon Linux 2023
* Browser version: Firefox

## Checklist
[] This issue fix need to be backported.
[] Does this issue have **security** implications?