调试为什么 nix 在 nixpkgs 二进制缓存中不必要地构建包？

我在 IRC 频道中问过这个问题，但我想我会在这里记录它以供我自己和其他人参考。

我有一个 shell.nix 文件需要很长时间才能构建，特别是它似乎正在构建一个HUnit包。

我的 shell.nix 文件如下所示：

{
 sources ? import ./nix/sources.nix
, compiler ? "ghc865" } :
let
  niv = import sources.nixpkgs {
    overlays = [
      (_ : _ : { niv = import sources.niv {}; })
    ] ;
    config = {};
  };
  pkgs = niv.pkgs;
  myHaskellPackages = pkgs.haskell.packages.${compiler}.override {
  };
in
myHaskellPackages.callCabal2nix "moscoviumorange" (./.) {}

并且源被固定到（使用 niv）：

{
    "niv": {
        "branch": "master",
        "description": "Easy dependency management for Nix projects",
        "homepage": "https://github.com/nmattia/niv",
        "owner": "nmattia",
        "repo": "niv",
        "rev": "88d6f20882b0422470acbcbf2d1b5f07e1d436f0",
        "sha256": "0wkvz4drnglmmdrz8q1i1yr2fqizpf96k1wq2rlhd8l8x1522izq",
        "type": "tarball",
        "url": "https://github.com/nmattia/niv/archive/88d6f20882b0422470acbcbf2d1b5f07e1d436f0.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
    },
    "nixpkgs": {
        "branch": "nixos-19.03",
        "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
        "homepage": "https://github.com/NixOS/nixpkgs",
        "owner": "NixOS",
        "repo": "nixpkgs-channels",
        "rev": "775fb69ed73e7cf6b7d3dd9853a60f40e8efc340",
        "sha256": "1w068b0ydw4c26mcjiwlzdfqcdk3rrwmfx4hxzgfhfwcz2nmh3if",
        "type": "tarball",
        "url": "https://github.com/NixOS/nixpkgs-channels/archive/775fb69ed73e7cf6b7d3dd9853a60f40e8efc340.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
    }
}

当我假设它们应该在 nixpkg 的二进制缓存中时，为什么 nix 会构建一堆包？

例如 nix-shell 的输出包括：

building '/nix/store/7011izw8f2xyvhqadrhnmapddyz61f29-HUnit-1.6.0.0.drv'...