Use Job Router feature flag from runner config instead of env variable directly

We currently directly use the job router feature flag value from the set environment variables (via os.Getenv) which bypasses the usual feature flags configuration mechanisms - in this case that's the runner config.

I've raised this limitation initially in !5945 (comment 3115818355).

The difficulty here is that the runner config is not yet parsed when we create the GitLab client (which we do differently when job router is enabled).