ci: clang: Use Redis remote storage for ccache

This commit updates the clang workflow to, when available, use Redis remote
storage backend for the ccache compilation cache data.

The Redis cache server is hosted in the Kubernetes cluster in which the
zephyr-runner pods run -- the Redis remote storage backend will be ignored
if the server is unavailable.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>