Fix the only:variables condition for the 'notify_slack' job

Otherwise, pipelines triggered from gitlab-org/gitlab try to notify Slack (fortunately there's no channel set): https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/337629807

The relevant part of the doc says:

With only, individual keys are logically joined by an AND:

(any of refs) AND (any of variables) AND (any of changes) AND (if Kubernetes is active)

– https://docs.gitlab.com/ee/ci/yaml/#onlyexcept-advanced