Self-hosted runner failing when trying to use new Gitlab runner authentication flow

The current code for acquiring a runnerToken attempts to fetch a token in the following order:

1. Use the legacy endpoint (`/projects/`
2. Use the new runner authentication endpoint (`/user/runners`)

However, in the legacy path, the legacy endpoint can return `undefined` for the token when legacy runner tokens are not enabled. This leads to [the null check](https://github.com/iterative/cml/blob/a3a66c74b218b08710f42bf599b066e37cb4cba7/src/drivers/gitlab.js#L154) entry to the new endpoint code path failing.