Use strip tool from darwin.cctools

Using the one from binutils makes a rust binary compiled with `-Cstrip=debuginfo` getting killed:

```
$ bazel-out/darwin-opt-exec-C7777A24/bin/external/rules_rust/util/process_wrapper/process_wrapper
Killed: 9
$ sudo dtruss bazel-out/darwin-opt-exec-C7777A24/bin/external/rules_rust/util/process_wrapper/process_wrapper
dtrace: failed to execute bazel-out/darwin-opt-exec-C7777A24/bin/external/rules_rust/util/process_wrapper/process_wrapper: Malformed Mach-o file
```

toolchains/rust/rust.bzl

@@ -29,7 +29,7 @@ let
           paths = [ pkgs.rustc ];
           nativeBuildInputs = [ pkgs.makeWrapper ];
           postBuild = ''
-            wrapProgram $out/bin/rustc --suffix PATH : ${{pkgs.binutils}}/bin
+            wrapProgram $out/bin/rustc --suffix PATH : ${{pkgs.darwin.cctools}}/bin
           '';
         }}
       else pkgs.rustc;