ci: Allow cancellation of redundant pipelines

Pipelines that are made redundant by a subsequent push are not getting cancelled even though we've enabled the corresponding setting in the Gitaly project. Seemingly, this is a feature that was introducen in GitLab 12.3 where jobs also need to declare that they are interruptible or otherwise GitLab won't honor that setting.

Mark all of our jobs as interruptible to stop wasting compute.