cargo: fix tests eval

Without the change `cargo` tests fail the eval as:

    $ nix build --no-link -f. cargo.tests
    error: attribute 'tests' missing

           at pkgs/development/compilers/rust/cargo.nix:22:15:

               21|
               22|   passthru = {
                 |               ^
               23|     rustc = rustc;

The test teried to pull `tests` attribute from `rustc`, which is a
wrapper without `tests`. Pull tests from unwrapped rust instead.

pkgs/development/compilers/rust/cargo.nix

@@ -21,7 +21,7 @@ rustPlatform.buildRustPackage.override {
 
   passthru = {
     rustc = rustc;
-    inherit (rustc) tests;
+    inherit (rustc.unwrapped) tests;
   };
 
   # Upstream rustc still assumes that musl = static[1].  The fix for