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`.
- API change: remove the `target` argument of `buildRustPackage`, the
target should always be in sync with the C/C++ compiler that is used.
- Gathering of binaries has moved from `buildPhase` to `installPhase`,
this simplifies the hook and orders this functionality logically
with the installation logic.
