CI: install xz-utils on Debian

Recently Debian testing started using xz compressed modules breaking the
CI. Upon a look, the xz-utils package and thus xz tool were missing.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

.github/workflows/tests.yml

@@ -66,7 +66,7 @@ jobs:
       if: matrix.distro.name == 'debian'
       run: |
         apt-get update -q
-        apt-get install -qy make linux-headers-amd64 linux-image-amd64 openssl
+        apt-get install -qy make linux-headers-amd64 linux-image-amd64 openssl xz-utils
 
     - name: Install Gentoo Linux dependencies
       if: matrix.distro.name == 'gentoo/stage3'