This adds a barebones CI-runner module with the following option:
`pub-solar.ci-runner.enable`
If enabled, this will start a systemd service on boot that runs
`drone-runner-exec`. The configuration expects you to have a file called
`secrets/drone-runner-exec-config` handled by agenix that gets put into
`/run/agenix/drone-runner-exec-config` and is owned by root.
This file should contain a configuration similar to the following:
```
CLIENT_DRONE_RPC_PROTO=https
CLIENT_DRONE_RPC_HOST=drone.company.com
CLIENT_DRONE_RPC_SECRET=super-duper-secret
```
