core/nixos: add architecture compat checks

[sstent]

SKIFF_CONFIG=pi/3,core/nixos

Boot fails to start NixOS as upstream NixOS image repo doesn't support ARMHF and this seems to bypass the trigger to build the image locally.

[paralin]

@sstent As a temporary solution you could try enabling arm64 in the config.txt for the pi 3, because pi3 actually does have the ability to run arm64 code