Recursive Nix is broken with --store

[L-as]

Describe the bug

addDependency uses setns which needs privileges to enter the mount namespace to add bind mounts. With --store when not run as root, this fails.

When using Recursive Nix with --store and root, it fails here instead for some yet unknown reason.

In practice, this means that any non-trivial derivation built inside the sandbox will fail as it causes addDependency to be called.

Steps To Reproduce

{ pkgs ? import <nixpkgs> {} }:

let
  hello-nix = pkgs.writeText "hello.nix" ''
    with import ${pkgs.path} {}; hello
  '';
in pkgs.runCommand "djE" {
  nativeBuildInputs = [ pkgs.nixUnstable ];
  requiredSystemFeatures = [ "recursive-nix" ];
} ''
  nix-build ${hello-nix}
  cp -L ./result $out
''

Expected behavior

It should work.

nix-env --version output

nix (Nix) 2.4pre20210922_bcd73eb

[L-as]

This bug seems to have existed from the start.