If you’re trying to use GitHub actions and Cachix to build aarch64 derivations, you can do this with
uses: docker/setup-qemu-action@v1
or similarly, run the following installation on ubuntu:
sudo apt-get update -q -y && sudo apt-get install -q -y qemu-system-aarch64 qemu-efi binfmt-support qemu-user-static
Then do your build with
nix-build --option system aarch64-linux --extra-platforms aarch64-linux