Geo: Verification worker should check health of shard

In gitlab-com/migration#568 (closed), we were seeing a lot of DeadlineExceeded errors. It turns out nfs-file-12 was broken:

irb(main):038:0> p = Project.find(4662084)
irb(main):039:0> Gitlab::Geo::ShardHealthCache.healthy_shard?(p.repository_storage)
=> false

I think this was because the Geo::RepositoryVerification::Secondary::SingleWorker (or the scheduler) wasn't paying attention to the health shard cache.