castor: enable parallels tests with nextest

pkgs/applications/networking/browsers/castor/default.nix

···
 
   postInstall = "make PREFIX=$out copy-data";
 
-  # Sometimes tests fail when run in parallel
-  dontUseCargoParallelThreads = true;
+  useNextest = true;
 
   meta = with lib; {
     description = "A graphical client for plain-text protocols written in Rust with GTK. It currently supports the Gemini, Gopher and Finger protocols";