Ericson2314: unrelated to the above, but for something like what you're saying with forAllSystem, we could add a `--apply-overlay` arg to nix installables which does the overlay application for you. so something like this would work:

```
$ nix build github:nixos/nixpkgs#legacyPackages.pkgsCross.aarch64-multiplatform --apply-overlay github:nixos/nix
```