checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
bindgen's tests rely on specific output from rustfmt that uses
unstable features, so breaks when we build rustfmt as stable.
Also, use a better way to specify the rustfmt executable.
