buildRustPackage: use checkType argument

The `checkType` argument of buildRustPackage was not used anymore
since the refactoring of `buildRustPackage` into hooks. This was
an oversight that is fixed by this change.

The check type can also be passed directly to cargoCheckHook using the
`cargoCheckType` environment variable.

pkgs/build-support/rust/hooks/cargo-check-hook.sh

@@ -15,7 +15,7 @@ cargoCheckHook() {
         threads=1
     fi
 
-    argstr="--${cargoBuildType} --target @rustTargetPlatformSpec@ --frozen";
+    argstr="--${cargoCheckType} --target @rustTargetPlatformSpec@ --frozen";
 
     (
         set -x