add nixos and darwin-nix modules

nixos-test.nix

@@ -0,0 +1,29 @@
+self: {
+  name = "nix-cache-login-nixos-module";
+
+  nodes.machine =
+    { ... }:
+    {
+      imports = [ self.nixosModules.default ];
+      services.nix-cache-login.enable = true;
+    };
+
+  testScript = ''
+    machine.wait_for_unit("multi-user.target")
+
+    # The module should install timer and service unit files for all users
+    machine.succeed("test -f /etc/systemd/user/nix-cache-login.timer")
+    machine.succeed("test -f /etc/systemd/user/nix-cache-login.service")
+
+    # wantedBy = ["timers.target"] should create this symlink
+    machine.succeed(
+        "test -L /etc/systemd/user/timers.target.wants/nix-cache-login.timer"
+    )
+
+    # Service unit should reference the correct subcommand
+    unit = machine.succeed("cat /etc/systemd/user/nix-cache-login.service")
+    assert "nix-cache-login refresh" in unit, (
+        f"ExecStart not found in service unit:\n{unit}"
+    )
+  '';
+}