ci: codecov: Use zephyrproject-rtos/action-s3-cache

This commit updates the Codecov workflow to use the 'action-s3-cache'
action from the zephyrproject-rtos organisation.

It also updates the action reference such that it uses a specific
version of the action instead of the latest master.

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