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.
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`.