support zstd for scanner and builder

[terefang]

use case: https://fedoraproject.org/wiki/Changes/Switch_RPMs_to_zstd_compression

• scanner already supports variant (de-)compressions but is missing zstd.
• builder needs to be updated
• ant-task needs to be updated

[terefang]

adding to that EL8 (8.2) also has been updated to support zstd: https://bugzilla.redhat.com/show_bug.cgi?id=1715799