add labels to distinguish piuparts system.tar.xz

piuparts lists two system.tar.xz (one with trixie as the outer container and one with the target chroot). This is confusing. Can you please add some label and explanation so users can understand what they need? Thanks to Helmut for explaining this to me.

They are also both named system.tar.xz when downloaded, would be great to give them different names.