With the shared system token, every time the Woodpecker host would
restart, a new Woodpecker agent registration would be created, because
the agent receives a unique ID on first connection using the system
token, which it couldn't store however, because it doesn't have a
writable config file in NixOS.
Use an agent token now, which doesn't require the agent to store a
unique ID in a wrtiable config, therefore not making it re-register.
Also see:
https://woodpecker-ci.org/docs/administration/agent-config#agent-registration
