tomb doesn't work with bash shell

[bricewge]

Issue description

tomb doesn't work under bash shell but it does with zsh andI don't understand the cause of it. I was working on #57946 when I encountered this.

Steps to reproduce

1. Start a bash sell with nix-shell --pure -p tomb zsh or env -i bash --noprofile --norc
2. Run tomb -h, it returns:

tomb [E] Missing required dependency pinentry.  Please install it.

3. Start a zsh shell in the same nix-shell
4. tomb -h now correctly return it's usage

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 4.19.28, NixOS, 18.09.2360.8751edf7b84 (Jellyfish)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.1.3`
 - channels(root): `"nixos-18.09.2360.8751edf7b84, nixos-hardware, nixpkgs-unstable-19.03pre169106.1a88aa9e0cd"`
 - channels(bricewge): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs`

[Mic92]

Does adding the pinentry package help?

[bricewge]

Unfortunately no. You can try it yourself with nix-shell --pure -p tomb pinentry --command "tomb -h"

[bricewge]

I don't think is still relevant any more, I have updated the PR with an install check which work.

So it seems that the issue only manifest itself when using nix-shell.