Update unpack_command to work with latest LAVA

This is the same fix as tests/apertis-test-cases!430 (merged)

LAVA version 2021.11 introduces a change in the way path 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