Launcher unpacks boot image.

The extraction was being performed by crun on glinux and
fetch_artifacts on gce, so it needs to be done in the launcher to stop
depending on crun.

fetch_artifacts should continue to unpack boot images on gce to ensure
the host packages remain capable of booting older builds.

Bug: 110478603
Test: local and gce
Change-Id: I0b925e700ef8b1693f7dc942bf3fa42b9f4daff8