docker: Ensure we use the cross toolchain for aarch64 on arm64

docker: Ensure we use the cross toolchain for aarch64 on arm64

We do not want to use the host toolchain for building our platforms in
CI (it is both too old, and would be inconsistent with our CI
practices). To do this we need to set the toolchain-prefix so that we
don't end up guessing "/opt/.../aarch64-linux-aarch64-linux-" as the
prefix.

Link: https://source.denx.de/u-boot/custodians/u-boot-dm/-/issues/32
Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>