.zsync files for the weekly live images

Hello Debian-Images team,

I've tested the .zsync files on [1].
They currently don't work, because the original filename 
(live-image-amd64.hybrid.iso) is embedded in those files.

As a fix, I see several possibilities, which do you prefer?

1) Turn off zsync
2) Keep zsync, and generate unique filenames

Option 1 would possibly increase the amount of traffic on 
get.debian.org. Are metrics available whether the .zsync files are used 
at all?
I haven't tested whether the zsync method will work properly for the 
live images, as they contain a huge squashfs file embedded in the iso 
file (so a compressed file inside another compressed file), so I don't 
know whether it will help in saving bandwidth.

Option 2 would mean that (with the current code) filenames can be 
generated with the structure 
`{something-we-decide}-{architecture}.hybrid.iso`, e.g. 
`debian-live-testing-gnome-amd64.hybrid.iso`. This is different from the 
current names on [1].
If we go for option 2, it will need (small) changes in the `live-build`, 
`live-setup` and `jenkins.debian.net` git repos.

With kind regards,
Roland Clobus

[1] https://get.debian.org/images/weekly-live-builds/amd64/iso-hybrid/