nixos-help: bundle the desktop item with the script

This is to ensure that whenever we install the desktop item we also have
the script installed. Prior to b02719a we always had the reference to
the script in the desktop item. Since desktop items are being copied to
home directories and thus "bit rod" over time that absolute path was
removed.

nixos/modules/misc/documentation.nix

@@ -40,8 +40,9 @@ let
       in scrubbedEval.options;
   };
 
-  helpScript = pkgs.writeShellScriptBin "nixos-help"
-    ''
+
+  nixos-help = let
+    helpScript = pkgs.writeShellScriptBin "nixos-help" ''
       # Finds first executable browser in a colon-separated list.
       # (see how xdg-open defines BROWSER)
       browser="$(
@@ -58,14 +59,22 @@ let
       exec "$browser" ${manual.manualHTMLIndex}
     '';
 
-  desktopItem = pkgs.makeDesktopItem {
-    name = "nixos-manual";
-    desktopName = "NixOS Manual";
-    genericName = "View NixOS documentation in a web browser";
-    icon = "nix-snowflake";
-    exec = "nixos-help";
-    categories = "System";
-  };
+    desktopItem = pkgs.makeDesktopItem {
+      name = "nixos-manual";
+      desktopName = "NixOS Manual";
+      genericName = "View NixOS documentation in a web browser";
+      icon = "nix-snowflake";
+      exec = "nixos-help";
+      categories = "System";
+    };
+
+    in pkgs.symlinkJoin {
+      name = "nixos-help";
+      paths = [
+        helpScript
+        desktopItem
+      ];
+    };
 
 in
 
@@ -249,8 +258,8 @@ in
 
       environment.systemPackages = []
         ++ optional cfg.man.enable manual.manpages
-        ++ optionals cfg.doc.enable ([ manual.manualHTML helpScript ]
-           ++ optionals config.services.xserver.enable [ desktopItem pkgs.nixos-icons ]);
+        ++ optionals cfg.doc.enable ([ manual.manualHTML nixos-help ]
+           ++ optionals config.services.xserver.enable [ pkgs.nixos-icons ]);
 
       services.mingetty.helpLine = mkIf cfg.doc.enable (
           "\nRun 'nixos-help' for the NixOS manual."