If you're running NixOS and want to use this template to build the Raspberry Pi 4 Image, you'll need to emulate an arm64 machine by adding the following to your NixOS configuration.
{
boot.binfmt.emulatedSystems = [ "aarch64-linux" ];
}
Then you will be able to run nix build .#images.pi4 and get a result you can
flash to an SD Card and boot.
After you've booted, you will be able to rebuild the nixosConfiguration on the
Pi. For example, by running nixos-rebuild --flake github:matthewcroughan/raspberrypi-nixos-example
Simply fork this repo and begin adding code to ./configuration.nix and allow
this basic configuration to become your own.