zsh HISTFILE is exported when it should not be

[ivan]

Issue description

The nixpkgs zsh module

nixpkgs/nixos/modules/programs/zsh/zsh.nix

Lines 192 to 195 in 6a73992

	# Don't export these, otherwise other shells (bash) will try to use same histfile
	SAVEHIST=${toString cfg.histSize}
	HISTSIZE=${toString cfg.histSize}
	HISTFILE=${cfg.histFile}

wants HISTFILE to not be exported, but HISTFILE is already exported at that point.

Steps to reproduce

Start a bash shell and observe zsh history get clobbered.

or

Run typeset -p HISTFILE and observe an export HISTFILE=...

or

Add typeset -p HISTFILE || true early to

nixpkgs/nixos/modules/programs/zsh/zsh.nix

Lines 182 to 183 in 6a73992

	''
	# /etc/zshrc: DO NOT EDIT -- this file has been generated automatically.

and observe export HISTFILE=/dev/null.

Technical details

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

• system: "x86_64-linux"
• host os: Linux 4.19.37-hardened, NixOS, 19.09.git.2f96f26 (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2.2
• channels(root): "nixos-18.09.1922.97e0d53d669"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

[ivan]

I kept observing an exported HISTFILE because it was still exported in the parent process. Killed tmuxes, logged out, and it's fine now.