fix nix-serve with hacky workaround

Author: Radvendii

tests/nixos/fetchers-substitute.nix

@@ -4,6 +4,26 @@
   nodes.substituter =
     { pkgs, ... }:
     {
+      # nix-serve is broken while cross-compiling in nixpkgs 25.11. It's been
+      # fixed since, but while we're pinning 25.11 we use this workaround.
+      nixpkgs.overlays = [
+        (
+          final: prev:
+          let
+            v25_11 = final.lib.versions.majorMinor final.lib.version == "25.11";
+            warning = final.lib.warnIf (
+              !v25_11
+            ) "remove the hack in fetchers-substitute.nix when updating nixpkgs from 25.11";
+          in
+          warning {
+            nix-serve = prev.nix-serve.override {
+              nix = prev.nix // {
+                libs.nix-perl-bindings = prev.nix.nix-perl-bindings ? null;
+              };
+            };
+          }
+        )
+      ];
       virtualisation.writableStore = true;
 
       nix.settings.extra-experimental-features = [