fixing steam in NixOS

configuration.nix

# all mirrors in pkgs/build-support/fetchurl/mirrors.nix for steamrt are currently down or returning 404s
# used https://gist.github.com/msteen/6741878f8919f8e2363869693e7ef085 as an example

{ config, pkgs, options, ... }:
let
  upstream =
    import (/home/djanatyn/repos/nixpkgs) { config = config.nixpkgs.config; };
in {
  imports = [ ./hardware-configuration.nix ];

  # nix.nixPath = options.nix.nixPath.default ++ [
  #   "nixos-19.09=/nix/var/nix/profiles/per-user/root/channels/nixos-19.09"
  # ];

  nixpkgs.overlays = [
    (self: super:
      with builtins; rec {
        mirrorsPath = self.path + /pkgs/build-support/fetchurl/mirrors.nix;
        customMirrors = import mirrorsPath // {
          steamrt = [ "https://mirrors.tuna.tsinghua.edu.cn/steamos/steamrt/" ];
        };

        customImport = scopedImport {
          import = path:
            if path == self.mirrorsPath then
              self.customMirrors
            else
              self.customImport path;
        };

        fetchurl = (self.customImport self.path { }).fetchurl;
      })
  ];

  nixpkgs.config.allowUnfree = true;

  nixpkgs.config = {
    packageOverrides = pkgs: {
      upstream = import upstream { config = config.nixpkgs.config; };
    };
  };