grafana: Fix disabled dashboards provisioning due to duplicates

Symlinking home.json to archive.json causes a duplicate, as both
dashboards have the same uid, and Grafana won't keep the dashboards
updated when there are duplicates[1]. Instead just change
default_home_dashboard_path to point to the archive.json dashboard.

[1] "dashboards provisioning provider has no database write permissions
     because of duplicates"