samples: net: wifi: Disable building on NS platform variants

The sample currently doesn't build for non-secure nRF platforms due to
mbed TLS misconfiguration - TFM enforces PSA crypto API, while the
default configuration for supplicant uses legacy crypto configuration.
In result, build fails due to unsatisfied dependencies. Therefore,
disable this variant from building temporarily to unblock the CI, until
the issue is resolved.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>