Update verify_deb to match zstd compressed debs

In the review of Ubuntu's merge of sbuild, it was suggested that I modify verify_deb, which we patch to handle verifying debs with zstd compression, so that it can match both xz (current Debian style) and zstd (current Ubuntu style) debs, such that the change can be contributed upstream.

This change modifies verify_deb to check the name of the data tarball inside the generated deb. I'd prefer to use something a bit more "high level" than ar (something from the dpkg suite of utilities) but I couldn't find anything that reported the compression type used in deb, hence why it's using a rather basic method.

For reference, #892664 is the bug tracking zstd compression of debs in Debian.