west.yml: point civetweb at the zephyrproject-rtos GH fork

We don't want to point anything outside of the Zephyr GitHub
organization. Let's point at the recently created zephyr fork instead.

Details on how this will be managed going forward, according to
standard project policy:

https://docs.zephyrproject.org/latest/guides/modules.html



Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>