tests: boards: nrf: nrfs: Build main image for app core only

Update this multi-core test to always run the `main` and `remote` images
on cpuapp and cpurad respectively.

This is to prepare the test for running with IronSide SE, in which case
keeping cpurad as the main board target wouldn't make as much sense,
because cpurad would have to be started by cpuapp.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>