gitSetupHook: init (#402320)

- A toggle has been added under `users.users.<name>.enable` to allow toggling individual users conditionally. If set to false, the user account will not be created.

- New hooks were added:

  - `writableTmpDirAsHomeHook`: This setup hook ensures that the directory specified by the `HOME` environment variable is writable.

  - `addBinToPathHook`: This setup hook checks if the `bin/` directory exists in the `$out` output path and, if so, adds it to the `PATH` environment variable.

  - `gitSetupHook`: This setup hook sets up a valid Git configuration, including the `user.name` and `user.email` fields.

<!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

## NixOS Wiki {#sec-release-25.05-wiki}

# shellcheck shell=bash

gitSetup () {

    GIT_CONFIG_GLOBAL=$(mktemp -t gitconfig.XXXXXX)

    export GIT_CONFIG_GLOBAL

    @gitMinimal@ config --global user.name GitSetupHook

    @gitMinimal@ config --global user.email GitSetupHook@nixpkgs.invalid

}

postHooks+=(gitSetup)

{

  lib,

  gitMinimal,

  makeSetupHook,

}:

makeSetupHook {

  name = "gitSetupHook";

  substitutions = {

    gitMinimal = lib.getExe gitMinimal;

  };

} ./gitSetupHook.sh