nixos: Fix it

nixos.nix

@@ -44,7 +44,7 @@ let
 
 in {
   options.ghc-bot.gitlabAccessToken = {
-    type = lib.types.str;
+    type = pkgs.lib.types.str;
     description = "GitLab access token for ghc-bot";
   };
   config.imports = [ triage-service test-tracking-service ];