[doc/ci] Require xz-utils for toolchain builds

Toolchains are distributed as .tar.xz, ensure that CI runners and our
users have support for it installed.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>

apt-requirements.txt

diff --git a/apt-requirements.txt b/apt-requirements.txt
index 8987540..19da59a 100644
--- a/apt-requirements.txt
+++ b/apt-requirements.txt
@@ -32,3 +32,4 @@
 tree
 xsltproc
 zlib1g-dev
+xz-utils