samples: echo_client: fix rw612_openthread_rcp_host build in CI

MONOLITHIC_NXP_NBU cannot be used in CI as it relies on firmware blobs
to be fetched.
Disabling the feature so the CI can pass.
Also tagging this test as build_only.

Signed-off-by: Axel Le Bourhis <axel.lebourhis@nxp.com>