Update download_command to work with latest LAVA

LAVA version 2021.11 introduces a change in the way paths in compressed
overlays are handled breaking Apertis tests.

Fix the issue by specifying the path to be use to extract the compressed
overlay.

Additional information:

https://gitlab.collabora.com/lava/lava/-/commit/07048f042c65202eb75ac874cc93eabd2d96cd54



Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

lava/common-boot-macros.yaml

@@ -18,5 +18,5 @@
     {% if tar_additional_options is not defined -%}
         {% set tar_additional_options = "-v" -%}
     {% endif -%}
-    tar {{tar_additional_options}} -C / -z -x
+    tar {{tar_additional_options}} -C /var/lib -z -x
 {%- endmacro %}