ci: add option to overwrite CPU arch

ci: add option to overwrite CPU arch

When loading Vulkan ICD file, it uses the CPU machine identifier to
load the correct one, in case multiple versions are installed.

This is fine if the machine where Mesa has been built and the machine
where the test is run are exactly the same. But this is not always the
case. As example, for armhf architecture, the machine where Mesa is
built is identified as `arm7hlf`, but the Raspberry Pi 4 is identified
as `armv7l`, so it will fail to load the ICD file, though both are
totally compatible.

This allow to define the architecture instead.

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8745>