An incomplete artifact fails without checking fallback sources when skip-verify is used

With skip-verify, the artifact downloader has no way of validating whether a fetched artifact is a functional Elastic Agent archive and stops after fetching the first source. If the artifact is malformed, it doesn't fail until either the package metadata or unpacking step, after we've already left the fallback source loop. Potentially valid sources are never tried afterward and the upgrade fails entirely.

The fallback failure in #16191 is caused by this. It's also relevant to the upcoming multiple download URI feature in #16097.