executor/MaRS: Store the runner id, and check that the tags list matches the list in gitlab

We used to use a token to update runners' details such as the list of tags. Since this is not possible with shared runners without requiring an admin token (which we will never get), I dropped this requirement.

We can however still verify that the tags are valid without privileges. All we need to know is the id of the runner: See https://gitlab.freedesktop.org/api/v4/runners/2798

So, let's store the runner id in MaRS, then verify that the list of tags match before exposing the machine on gitlab :)