ci: document arm oddity in build-rules

ci: document arm oddity in build-rules

There's an oddity in the .lava-test:amd64 build rules; we depend on and
use the ARM images instead of the AMD64 images. The reason for this is
kind of silly; we need the Docker image to match the architecture of the
runner, which happens to be ARM.

So this isn't at all about the target architecture, but more of a Docker
detail.

Hopefully documenting this will prevent others from spending time being
puzzled about this in the future.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8967>