gitlab-ci: Rename container install scripts to match job names (better)

Cleans up .gitlab-ci/ a little, and allows using a single DEBIAN_EXEC
line for all container jobs.

v2:
* Use lava_arm.sh instead of arm_lava.sh for consistency with v2 of the
  previous change

Reviewed-by: Eric Anholt <eric@anholt.net> # v1
Reviewed-by: Eric Engestrom <eric@engestrom.ch>