The Github runner is now more limited and does not accept the standardzephyrprojectrtos/ci image. Use the ci-base image and manually installthe few extra dependencies we need via action-setup-zephyr.This brings an additional path issue: the west workspace will beinitialized in $GITHUB_WORKSPACE/.., so we need to move the repocheckout to a subfolder and then move it to the right place.

Limit the list of HALs to only those needed by the boards in boards.txt.Copied verbatim from bootstrap.sh.

ccache takes a long time to save, almost as much as downloadingthe archive every time, so skip it.