runInLinuxVM: Ensure tools requiring /etc/passwd work

[samueldr]

This includes, but is not limited to:

• whoami
• nix >= 2.3.1

See

• NixOps libvirtd fails since username can't be detected #71157
• [libvirtd] Unable to deploy to libvirtd - "error: cannot figure out user name" nixops#1216
• libvirtd-image fails to be created nix-community/nixops-libvirtd#5

Motivation for this change

Fix a system regression in 19.09, where nixops will not work using libvirtd, as pointed out personally to me by @JoeDupuis.

This fix, though, is a much broader fix, and could fix any tool that tries to identify the user in this step.

Things done

Tested using this:

let
  pkgs = import <nixpkgs> {};
  script = name: cmd: pkgs.runCommand "runInLinuxVM--lack-of-user--${name}" {
    buildInputs = with pkgs; [
      utillinux
      shadow
    ];
  } ''
    (PS4=" $ "; set -x
      ${cmd}
    )
  '';
  test = name: cmd: pkgs.vmTools.runInLinuxVM (script name cmd);
in
  [
    (test "id" "id | grep root")
    (test "whoami" "whoami")
    # https://github.com/NixOS/nixpkgs/issues/71157
    (test "nix" "${pkgs.nix.out}/bin/nix-store --version")
  ]

You can verify that without this fix, both on unstable, and 19.09 this fails in three different, but coherent ways. With this fix applied, it works.

This should be backported to 19.09, in my opinion, as this is a system breakage from a minor bump from nix in the nixops ecosystem.

This has to be backported to 20.03 if accepted in master.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.