ci: codecov: Use zephyr-runner v2

This commit updates the codecov workflow to use the new zephyr-runner v2 CI
runner deployment.

It also updates the workflow to use the `ci-repo-cache` Docker image, which
includes the Zephyr repository cache, because the node level repository
cache is no longer available in the zephyr-runner v2.

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