Don't run robustness tests on user forks

[ivanvc]

What would you like to be added?

This is something I have spotted for a long time (and can potentially flood user notifications). The configuration for the robustness tests workflow asks for a runner that regular users don't have access to (and it's also trying to run the tests on actuated ARM64 runners, which regular users don't have access to).

We should run these tests only for the etcd-io/etcd (and etcd-io/bbolt) repository, as we have configured other runners.

Note I know that the plan is to eventually relocate all Robustness tests to the Prow infrastructure, but in the meantime, it floods with notifications, which decreases the contributor experience.

Why is this needed?

To save users from receiving notifications that jobs are failing due to timeout trying to secure a runner to do the job. Improve ContribEx.