nix-serve: pin nix to nixVersions.nix_2_18 (previous stable) due to breaking changes

Author: fabianhjr

pkgs/top-level/all-packages.nix

@@ -38044,7 +38044,9 @@ with pkgs;
 
   nixpkgs-review = callPackage ../tools/package-management/nixpkgs-review { };
 
-  nix-serve = callPackage ../tools/package-management/nix-serve { };
+  nix-serve = callPackage ../tools/package-management/nix-serve {
+    nix = nixVersions.nix_2_18;
+  };
 
   nix-serve-ng = haskell.lib.compose.justStaticExecutables haskellPackages.nix-serve-ng;