Create a verification for cached_task artifact expiry

[ahal]

Gecko recently started expiring unimportant artifacts before the task expiry to save on storage costs. The problem is that some tasks (mainly cached tasks like toolchains, fetches and docker-images) only exist to provide long lived artifacts. And sometimes a mistake is made where one of these artifacts is set to expire prior to the task. When this happens, it causes failures in downstream tasks that end up depending on the cached tasks, but then get a 404 when they go to download it.

Let's add a new verification that will prevent this from happening, at least for cached tasks.