NixOS modules should not set environment variables to nix store paths

[amozeo]

Describe the bug

When updating system using nixos-rebuild switch or nixos-rebuild test, environment variables usually are not being updated in running processess and user sessions, and in order for them to be updated, a session relog or system restart would need to happen.

Thus, environement variables could contain outdated store path, and it's the reason why nix store paths should not be used in environment variables and be actively discouraged.

for configuration files, /etc directory could be used as an alternative.

I have found that these NixOS modules use nix store paths in environment variables

• nixos/modules/config/shells-environment.nix:142 - fcitx5Package
• nixos/modules/programs/freetds.nix:45 - pkgs.freetds
• nixos/modules/programs/less.nix:118 default value of lessopen has store path pkgs.lesspipe
• nixos/modules/programs/plotinus.nix:31 - pkgs.plotinus
• nixos/modules/programs/plotinus.nix:32 - pkgs.plotinus
• nixos/modules/programs/ssh.nix:342 - default value of askPassword has store path pkgs.x11_ssh_askpass
• nixos/modules/services/desktops/gnome/tracker.nix:61 - subcommandPackagesTree
• nixos/modules/services/network-filesystems/kubo.nix:288 - fakeKuboRepo

This issue was originally reported in https://github.com/NixOS/nixpkgs/issues/311207 however I believe a separating it from that issue is better.

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[Atemu]

Note that this only concerns sessions variables; env vars that are part of some long-running "session" that keeps its initial set of environment variables such as the user session.

Using nix store paths in env vars of i.e. systemd system services however is fine as they are restarted automatically upon activation when anything has changed.

Note again that the same is not true for user services however; they are not restarted upon activation.