nixos/tools: make desktopConfiguation types.listOf types.lines

This fixes #108124.

nixos/modules/installer/tools/tools.nix

@@ -87,8 +87,8 @@ in
 
     desktopConfiguration = mkOption {
       internal = true;
-      type = types.str;
-      default = "";
+      type = types.listOf types.lines;
+      default = [];
       description = ''
         Text to preseed the desktop configuration that <literal>nixos-generate-config</literal>
         saves to <literal>/etc/nixos/configuration.nix</literal>.

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -197,12 +197,12 @@ in
   config = mkMerge [
     (mkIf (cfg.enable || flashbackEnabled) {
       # Seed our configuration into nixos-generate-config
-      system.nixos-generate-config.desktopConfiguration = ''
+      system.nixos-generate-config.desktopConfiguration = [''
         # Enable the GNOME 3 Desktop Environment.
         services.xserver.enable = true;
         services.xserver.displayManager.gdm.enable = true;
         services.xserver.desktopManager.gnome3.enable = true;
-      '';
+      ''];
 
       services.gnome3.core-os-services.enable = true;
       services.gnome3.core-shell.enable = true;

nixos/modules/services/x11/desktop-managers/plasma5.nix

@@ -184,12 +184,12 @@ in
   config = mkMerge [
     (mkIf cfg.enable {
       # Seed our configuration into nixos-generate-config
-      system.nixos-generate-config.desktopConfiguration = ''
+      system.nixos-generate-config.desktopConfiguration = [''
         # Enable the Plasma 5 Desktop Environment.
         services.xserver.enable = true;
         services.xserver.displayManager.sddm.enable = true;
         services.xserver.desktopManager.plasma5.enable = true;
-      '';
+      ''];
 
       services.xserver.desktopManager.session = singleton {
         name = "plasma5";

nixos/tests/nixos-generate-config.nix

@@ -11,12 +11,12 @@ import ./make-test-python.nix ({ lib, ... } : {
       }
     '';
 
-    system.nixos-generate-config.desktopConfiguration = ''
+    system.nixos-generate-config.desktopConfiguration = [''
       # DESKTOP
       # services.xserver.enable = true;
       # services.xserver.displayManager.gdm.enable = true;
       # services.xserver.desktopManager.gnome3.enable = true;
-    '';
+    ''];
   };
   testScript = ''
     start_all()