nh: remove PATH assignment in nh-clean systemd unit

Runs without additions to path.

also:

- can't use $PATH in Environment=
- should add bin directory to path not the exe

modules/programs/nh.nix

@@ -74,8 +74,7 @@ in {
         Service = {
           Type = "oneshot";
           ExecStart =
-            "exec ${lib.getExe cfg.package} clean user ${cfg.clean.extraArgs}";
-          Environment = "PATH=$PATH:${config.nix.package}";
+            "${lib.getExe cfg.package} clean user ${cfg.clean.extraArgs}";
         };
       };