Fix BlockBlobClient cast error downloading from job

[directhex]

I make extensive use of "Download artifact from build -> Upstream build that triggered this job". As of today, this throws:

com.azure.storage.blob.specialized.BlockBlobClient cannot be cast to com.azure.storage.file.share.ShareFileClient

This doesn't work with the current mix of Azure plugin dependencies on an up-to-date Jenkins install. The core issue is when the download items are enumerated, they are of type BlockBlobClient not BlobClient - the instanceof check therefore falls through into the ShareFileClient case, and that's not a valid cast.

Instead, check for the parent type of both BlobClient and BlockBlobClient, BlobClientBase. This is valid for the current case and the old case, so should be safe for various combinations of dependency versions.

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