Hi RomMon,
I'm not sure if you're seeing the same problem as me, but I'm getting the same error when building for the Raspberry Pi and it's stopping at: UNPACK kernel-firmware
When I trace back the output I can see that the build is attempting to download an xz file, and gets redirected to a gz file instead.
It saves the file as an xz file and try's to decompress it as an xz file.
I couldn't see any downloads happening in your log but the issue seems similar.
If you can check the file by running the file command on it, it'll tell you the file type (I'm getting "gzip compressed data" which is causing the problem).
Unfortunately this only diagnoses the problem, I'm unsure how to fix it, it only surfaced for me yesterday when I kicked off a new build.
Hopefully it'll simply 'go away', but if I find a fix I'll pass it on.
Regards,
B.