home-environment: optionally empty PATH in activation
home-environment: optionally empty PATH in activation

This adds the option `home.emptyActivationPath` that, when enabled,
will cause the activation script to ignore the calling user's `PATH`.
The option is disabled by default to match current behavior but the
intent is to change this in the future to reduce risk of accidental
dependencies of the environment.

rycee/home-manager,rycee/home-manager,rycee/home-manager