А, вообще, что это за ошибка, и причём здесь xz?
99% [1 pkglist xz 0] [Waiting for headers]xz: (stdin): File format not recognized
Ошибка http://ftp.altlinux.org p10/branch/x86_64/classic pkglist
Sub-process xz returned an error code (1)
E: Failed to fetch http://ftp.altlinux.org/pub/distributions/ALTLinux/p10/branch/x86_64/base/pkglist.classic Sub-process xz returned an error code (1)
E: Some index files failed to download. They have been ignored, or old ones used instead.