ci: Disable shader cache on vulkan CI runs.

ci: Disable shader cache on vulkan CI runs.

I found it to be flaky in freedreno CI, and tracked down the issue to
parallel-deqp-runner needing to manage the shader cache
(https://gitlab.freedesktop.org/mesa/parallel-deqp-runner/-/merge_requests/13).
Until we fix that in the runner, disable it.  This should matter less now
that we prebuild the SPIRV, though.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5266>