Prior to this patch:
$ nix-instantiate --eval -E '
> with import ./. {
> localSystem.config = "aarch64-unknown-linux-musl";
> };
> (nixos {}).config.nixpkgs.localSystem.config
> '
"aarch64-unknown-linux-gnu"
Because only the system triple was being passed through, the Musl part
of the system specification was lost. This patch fixes various
occurrences of NixOS evaluation when a Nixpkgs evaluation is already
available, to pass through the full elaborated system attribute set,
to avoid this loss of precision.
c3c0dd00d8