fix(assets/init): force config.yml symlink to restart existing instances.

My runner instances don't survive a clean restart of the host. Docker (with the autostart feature) tries to start the instances but they immediately exit.

```sh
root@si-gitlab:~# docker ps -a
CONTAINER ID        IMAGE                               COMMAND               CREATED             STATUS                      PORTS                                   NAMES
81e85e03a30c        sameersbn/gitlab-ci-runner:latest   /app/init app:start   39 minutes ago      Exited (1) 9 minutes ago                                            gitlab-ci-runner-3                
```

The reason can be found in the log: `failed to create symbolic link 'config.yml': File exists` (the init script runs with `set -e` so it will abort at this point).

This tiny PR should fix the problem.