Impure nix-shell: ensure PATH prioritises buildInputs over existing env

1

I’m facing issues resulting from my env PATH being prioritised over the path from buildInputs in an impure nix-shell, which seems incorrect, since the intention of an impure shell is (always?) to be sure to use its explicit buildInputs over possibly existing versions.

As a workaround I’m now explicitly setting the path of sub-envs like this:

{ pkgs ? import <nixpkgs> { config.documentation = { man.enable = false; doc.enable = false; info.enable = false; }; }
, unstable ? import <unstable> { config.documentation = { man.enable = false; doc.enable = false; info.enable = false; }; }
}:

let
  pywpkgs = unstable.python310.withPackages (p: with p; [ pandoc-xnos openpyxl ]);
  rbwpkgs = unstable.ruby_3_1.withPackages (p: with p; [ pandocomatic ]);
  hswpkgs = unstable.haskellPackages.ghcWithPackages (h: with h; [
    pandoc pandoc-crossref pandoc-include-code pandoc-plot
  ]);
in

pkgs.mkShell {
  nativeBuildInputs = with pkgs; [
    (texlive.combine { inherit (texlive)
        scheme-small xcolor koma-script nth marvosym fontawesome multirow placeins enumitem tcolorbox
        lualatex-math environ lastpage titlesec advdate
        collection-langgerman collection-langenglish
    ;})
  ]
  ++ (with unstable; [
    pandoc-drawio-filter pandoc-include pandoc-plantuml-filter pandoc-lua-filters
  ])
  ++ [ hswpkgs pywpkgs rbwpkgs ]; # proxy vars to get the out path for the complete env

  # needed for LaTeX \today to function!
  shellHook = ''
    export SOURCE_DATE_EPOCH="$(date +%s)"
    export FORCE_SOURCE_DATE=1
    export PATH=${pywpkgs.out}/bin:${rbwpkgs.out}/bin:${hswpkgs.out}/bin''${PATH:+:''${PATH}}
  '';
}

but that is a lot of boilerplate for obvious expected default functionality. Am I doing something wrong or is there a better way?