zsh: fix trailing slash breaking zshenv

[Buttars]

Description

If a user has a trailing / in their user.users.<user>.home value with the zsh enabled the module fails to properly check the equality of dotDir and the home path.

The libs.nix module is stripping the trailing / from the dotDirAbs and dotDirRel and not for homeDir.

You can see the how the values are evaluated by running this script:

nix eval --json --impure --expr '
let
  flake   = builtins.getFlake ("path:" + toString ./.);
  libPkgs = if flake.inputs ? nixpkgs then flake.inputs.nixpkgs.lib else import <nixpkgs/lib> {};
  user    = builtins.getEnv "USER";
  host    = builtins.getEnv "HOSTNAME";

  hmCfg =
    if flake ? darwinConfigurations then
      let
        dcs = flake.darwinConfigurations;
        machineKey =
          if host != "" && builtins.hasAttr host dcs
          then host
          else builtins.head (builtins.attrNames dcs);
        hmUsers = dcs.${machineKey}.config.home-manager.users;
      in
        if builtins.hasAttr user hmUsers
        then hmUsers.${user}
        else throw "User ${user} not found in home-manager.users for ${machineKey}"
    else
      let
        hcs = flake.homeConfigurations;
        keys = builtins.attrNames hcs;
        matches = builtins.filter (k: (hcs.${k}.config.home.username or "") == user) keys;
        key = if matches != [] then builtins.head matches else builtins.head keys;
      in
        hcs.${key}.config;

  hmSrc = flake.inputs.home-manager.outPath;
  vals  = import (hmSrc + "/modules/programs/zsh/lib.nix") { config = hmCfg; lib = libPkgs; };
in { inherit (vals) homeDir dotDirAbs dotDirRel; }
' | jq

@lzpreslav actually mentioned this on the PR a few months back.
#6089 (comment)

Relevant LOC:
https://github.com/nix-community/home-manager/blob/master/modules/programs/zsh/lib.nix#L6
https://github.com/nix-community/home-manager/blob/master/modules/programs/zsh/lib.nix#L69
https://github.com/nix-community/home-manager/blob/master/modules/programs/zsh/lib.nix#L99-L100
https://github.com/nix-community/home-manager/blob/master/modules/programs/zsh/default.nix#L415

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

[Buttars]

If someone could guide me on writing tests for this I would appreciate it. I only see tests testing the output of the module and not any unit tests.

[Buttars]

@khaneliman