nixos/specialisation: add isSpecialisation option to know if we're a specialisation

nixos/modules/system/activation/no-clone.nix

@@ -5,4 +5,5 @@ with lib;
 {
   boot.loader.grub.device = mkOverride 0 "nodev";
   specialisation = mkOverride 0 {};
+  isSpecialisation = mkOverride 0 true;
 }

nixos/modules/system/activation/specialisation.nix

@@ -23,6 +23,12 @@ let
 in
 {
   options = {
+    isSpecialisation = mkOption {
+      type = lib.types.bool;
+      internal = true;
+      default = false;
+      description = "Whether this system is a specialisation of another.";
+    };
 
     specialisation = mkOption {
       default = { };