As home-manager does not restart the `sops-nix` unit automatically
a snippet to instruct home-manager to do so is added.

Home-manager could be instructed to restart the user service from the
sops-nix home-manager module instead. Usually home-manager restarts
units which changed. Since the sops-nix unit does not change when
secrets change this does not trigger automatically.

There are two options:
- let sops-nix home-manager module compute a chained hash over all
  secrets and place it inside the unit file, so it changes every time
  the secrets change
- use X-SwitchMethod and X-Restart-Triggers
  See nix-community/home-manager#3865