nixos/activation-script: disable userActivationScripts when system is not activatable

nixos/modules/system/activation/activation-script.nix

···
       source ${config.system.build.earlyMountScript}
     '';
 
-    systemd.user = {
+    systemd.user = lib.mkIf config.system.activatable {
       services.nixos-activation = {
         description = "Run user-specific NixOS activation";
         script = config.system.userActivationScripts.script;