If the `home-manager` module is enabled then check if the
`home-manager` package is installed using `nix-env -i` and if so then
it is automatically uninstalled before the new package environment,
which includes home-manager, is installed.
This module is a module to install and configure the home-manager
tool. By managing the home-manager tool through the Home Manager
module system it will be installed/updated on configuration
activation.
