mipsel-linux stdenv does not evaluate from flake

[ehmry]

Describe the bug

The flake output legacyPackages.mipsel-linux.stdenv does not eval. It would be preferable if it did, because then flakes could map over all the outputs of the Nixpkgs flake, like this:

{ self, nixpkgs }: {
  overlay = final: prev: { };
  legacyPackages = nixpkgs.lib.attrsets.mapAttrs
    (sys: pkgs: pkgs.extend self.overlay)
    nixpkgs.legacyPackages;
}

Error message is error: attribute 'busybox' missing from pkgs/stdenv/linux/bootstrap-tools/default.nix:6.

Steps To Reproduce

Steps to reproduce the behavior:

1. nix build nixpkgs#legacyPackages.mipsel-linux.stdenv

Expected behavior

A cleaner error, a broken package, or no mipsel in flake.nix.

Notify maintainers

@viric