Why can't I build without the nix-on-droid cachix?

[shelvacu]

I enabled aarch binfmt with boot.binfmt.emulatedSystems = [ "aarch64-linux" ]; and when I try to build I get this:

# nix build --impure .#nixOnDroidConfigurations.default.activationPackage
trace: warning: The type `types.string` is deprecated. See https://github.com/NixOS/nixpkgs/pull/66346 for better alternative types.
error:
       … while calling the 'derivationStrict' builtin

         at /builtin/derivation.nix:9:12: (source not available)

       … while evaluating derivation 'nix-on-droid-generation'
         whose name attribute is located at /nix/store/lr42p9vxg420i8yfqadqp1w9wxdmdci2-source/pkgs/stdenv/generic/make-derivation.nix:348:7

       … while evaluating attribute 'buildCommand' of derivation 'nix-on-droid-generation'

         at /nix/store/lr42p9vxg420i8yfqadqp1w9wxdmdci2-source/pkgs/build-support/trivial-builders/default.nix:87:14:

           86|       enableParallelBuilding = true;
           87|       inherit buildCommand name;
             |              ^
           88|       passAsFile = [ "buildCommand" ]

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: path '/nix/store/yrrs22jsl1y8niwzs2vvk0vblicr3903-proot-termux-static-aarch64-unknown-linux-android-unstable-2023-05-13' does not exist and cannot be created

However, if I add https://app.cachix.org/cache/nix-on-droid to the substituters with the proper key, it works fine. Where did cachix get it from?

[shelvacu]

You can add this to your own cache with nix copy github:nix-community/nix-on-droid#packages.aarch64-linux.prootTermux on an x86 machine with boot.binfmt.emulatedSystems = [ "aarch64-linux" ];

Otherwise this is answered in #332 (comment)