Clang can't find headers when cross-compiling back to same platform

[thenhnn]

Steps To Reproduce

Steps to reproduce the behavior:

1. build following expression on latest nixos-unstable:

let
  nixpkgs = builtins.getFlake "nixpkgs";
  pkgs = import nixpkgs {
    localSystem = "x86_64-linux";
    crossSystem = {
      system = "x86_64-linux";
      useLLVM = true;
      linker = "lld";
    };
  };
in
  pkgs.stdenv.mkDerivation {
    phases = ["build"];
    name = "test";
    build = ''
      echo "#include <iostream>" | $CXX -xc++ -E - > $out
    '';
  }

(It works, the problem is in something else)

Build log

test-x86_64-unknown-linux-gnu> Running phase: build
test-x86_64-unknown-linux-gnu> <stdin>:1:10: fatal error: 'iostream' file not found
test-x86_64-unknown-linux-gnu>     1 | #include <iostream>
test-x86_64-unknown-linux-gnu>       |          ^~~~~~~~~~
test-x86_64-unknown-linux-gnu> 1 error generated.

Additional context

Probably regression from #264865?

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 6.6.30, NixOS, 24.05 (Uakari), 24.05.20240507.b211b39`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.18.2`
 - nixpkgs: `/nix/store/f0ssy3p6898x9k880l23mhd0fl6rvrb2-source`

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[thenhnn]

Ooops. Sorry, it was XY problem. I will create other issue.