Force xz compression for satdep.deb

This ensures non-Debian systems with different default compression algorithms set for dpkg-deb can still run autopkgtest for Debian targets. This is specially true for the Ubuntu case, where zstd is now the default compression algorithm [1] and one can no longer run autopkgtest for a Debian target [2].

While proposing this change in Ubuntu [2], it was suggested that this patch should be forwarded here. While this is, at this time, Ubuntu specific, this should not change the current behavior of autopkgtest (but could add technical debt). ITOH, it allows Ubuntu users to keep running autopkgtest for Debian targets. Thoughts?

[1] https://bugs.launchpad.net/ubuntu/+source/dpkg/+bug/1923845 [2] https://bugs.launchpad.net/ubuntu/+source/autopkgtest/+bug/1947810