fix: ensure grafana does not install plugins from the internet

[mjnagel]

Description

The Grafana 11.3.0 update contained a new default pre-install plugin. While this plugin would be nice, the method of installation involves reaching out to grafana.com on startup to download and install the plugin. To workaround this for the time being this PR disables the pre-install functionality.

In reviewing you should notice that before the change there are log failures reaching out to grafana:

logger=plugin.backgroundinstaller t=2024-11-07T18:42:14.912141481Z level=info msg="Installing plugin" pluginId=grafana-lokiexplore-app version=
logger=plugin.backgroundinstaller t=2024-11-07T18:42:14.939302134Z level=error msg="Failed to install plugin" pluginId=grafana-lokiexplore-app version= error="Get \"https://grafana.com/api/plugins/grafana-lokiexplore-app/versions\": read tcp 10.42.0.36:49118->34.120.177.193:443: read: connection reset by peer"

After this change those failures do not exist.

Related Issue

Related to https://github.com/defenseunicorns/uds-core/issues/992

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Other (security config, docs update, etc)

Checklist before merging

• [x] Test, docs, adr added or updated as needed
• [x] Contributor Guide followed