drm/ci: export kernel config

Export the resultant kernel config, making it easier to verify if the
resultant config was correctly generated.

Suggested-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Link: https://lore.kernel.org/r/20231024004525.169002-9-helen.koike@collabora.com


Signed-off-by: Maxime Ripard <mripard@kernel.org>