ci: Only run 'e2e:test-on-gdk' on approved MRs

What does this MR do and why?

Running E2E tests against GDK can delay predictive pipelines by 13 minutes (e.g. https://gitlab.com/gitlab-org/gitlab/-/tracing/60df9ebd-b9a4-e640-d39d-43812c39e534):

In this MR, I propose only running e2e:test-on-gdk after an MR has been approved (i.e. when it as the pipeline:mr-approved label).