Check for .tar.xz instead of .tar.gz when publishing Debian package
Check for .tar.xz instead of .tar.gz when publishing Debian package

In #726 we moved to a more modern Debian package format but failed to
take into account the necessary changes to the package-deb script,
which now needed to expect a `.tar.xz` instead of a `.tar.gz`.

github/backup-utils,github/backup-utils