nixos-container: fix destroy with immutable /var/empty

fixes #21573

pkgs/tools/virtualization/nixos-container/nixos-container.pl

@@ -248,6 +248,7 @@ if ($action eq "destroy") {
 
     safeRemoveTree($profileDir) if -e $profileDir;
     safeRemoveTree($gcRootsDir) if -e $gcRootsDir;
+    system("chattr", "-i", "$root/var/empty") if -e $root;
     safeRemoveTree($root) if -e $root;
     unlink($confFile) or die;
 }