Cache is not thread safe

[lcarva]

The EC cache is used to avoid fetching the same image layers from the registry more than once. However, this is not thread safe.

A user reported random redhat_manifests.redhat_manifests_missing violations that should've not occurred. This can only be reproduced when validating multiple images that share layers.

Disabling the cache avoids the issue altogether.

Let's either make the cache safe, or pull the plug on it.