[200$ bounty] download and decompress xz images

[incognitojam]

we have a separate step to create .gz images for the flash tool - it should use the .xz images made for openpilot and agnos.py

See here https://github.com/commaai/flash/blob/8d9982cc1bdfe04ffc59dee87fb11fd77f811d79/src/workers/image.worker.js#L134