nh: fix clean option behaviour for Darwin

tests/modules/programs/nh/linux/config.nix

@@ -0,0 +1,35 @@
+{ config, pkgs, ... }:
+{
+  programs.nh = {
+    enable = true;
+    package = config.lib.test.mkStubPackage { version = "4.0.0"; };
+
+    flake = "/path/to/flake";
+
+    clean = {
+      enable = true;
+      dates = "daily";
+    };
+  };
+
+  nmt.script = ''
+    unitDir=home-files/.config/systemd/user
+    timerFile=$unitDir/nh-clean.timer
+
+    assertFileExists $timerFile
+    assertFileContent $timerFile ${pkgs.writeText "timer-expected" ''
+      [Install]
+      WantedBy=timers.target
+
+      [Timer]
+      OnCalendar=daily
+      Persistent=true
+
+      [Unit]
+      Description=Run nh clean
+    ''}
+    assertFileExists $unitDir/timers.target.wants/nh-clean.timer
+
+    assertFileRegex home-path/etc/profile.d/hm-session-vars.sh 'NH_FLAKE="/path/to/flake"'
+  '';
+}