Hi,

First, thanks for your work on sbuild, the more the time passes, the
more I enjoy this tool.

In config.pl, there are replacement placeholders for arch: %a and
release: %r. Could a % placeholder be made for the debian-derivative
flavor?

The idea is that I'm now working on both ubuntu and debian env, and
unifying my configs would be great. One of the limitations I have is for
autopkgtest selector for lxd/incus env.

Thanks <3