mdbook: add nix to passthru.tests

pkgs/tools/text/mdbook/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, rustPlatform, CoreServices }:
+{ lib, stdenv, fetchFromGitHub, nix, rustPlatform, CoreServices }:
 
 rustPlatform.buildRustPackage rec {
   pname = "mdbook";
@@ -21,6 +21,12 @@ rustPlatform.buildRustPackage rec {
   # is released.
   logLevel = "info";
 
+  passthru = {
+    tests = {
+      inherit nix;
+    };
+  };
+
   meta = with lib; {
     description = "Create books from MarkDown";
     homepage = "https://github.com/rust-lang/mdBook";