tests.makeBinaryWrapper: avoid same-triple cross

We shouldn't try compiling to aarch64-unknown-linux-gnu from
aarch64-unknown-linux-gnu, because that tends to confuse things and is
not representative of actual cross compilation usage.

pkgs/test/make-binary-wrapper/default.nix

···
     "prefix"
     "suffix"
   ] makeGoldenTest // lib.optionalAttrs (! stdenv.isDarwin) {
-    cross = pkgsCross.aarch64-multiplatform.callPackage ./cross.nix { };
+    cross = pkgsCross.${if stdenv.buildPlatform.isAarch64 then "gnu64" else "aarch64-multiplatform"}.callPackage ./cross.nix { };
   };
 in