nixos/home-assistant: make service reloadable

This allows quick reloads using the following trick:

$(nix-build --show-trace --no-out-link \
  -E '(with import <nixpkgs/nixos> {};
       pkgs.writeScript "update-hass-config"
       config.systemd.services.home-assistant.preStart)')
systemctl reload home-assistant

nixos/modules/services/misc/home-assistant.nix

@@ -240,6 +240,7 @@ in {
       '');
       serviceConfig = {
         ExecStart = "${package}/bin/hass --config '${cfg.configDir}'";
+        ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
         User = "hass";
         Group = "hass";
         Restart = "on-failure";