gitlab-ci: Use separate arm64 build/test docker images

gitlab-ci: Use separate arm64 build/test docker images

The image used for test jobs is only about 1/6 as big as before, which
may help avoid some issues with some of the test boards.

Inspired by https://gitlab.freedesktop.org/mesa/mesa/issues/2046 .

v2:
* Leave LIBDRM_VERSION at 2.4.99 (Daniel Stone)
* Delete more build artifacts from dEQP tree (Daniel Stone)
v3:
* Set LD_LIBRARY_PATH for ldd

Acked-by: Daniel Stone <daniels@collabora.com> # v2
Reviewed-by: Eric Anholt <eric@anholt.net> # Except for the ldd line