buildRustPackage: factor out check phase to cargoCheckHook

API change:

`cargoParallelTestThreads` suggests that this attribute sets the
number of threads used during tests, while it is actually a boolean
option (use 1 thread or NIX_BUILD_CORES threads). In the hook, this
is replaced by a more canonical name `dontUseCargoParallelTests`.

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

@@ -39,7 +39,7 @@ rustPlatform.buildRustPackage rec {
   postInstall = "make PREFIX=$out copy-data";
 
   # Sometimes tests fail when run in parallel
-  cargoParallelTestThreads = false;
+  dontUseCargoParallelThreads = 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";