Merge pull request #35338 from dtzWill/feature/release-cross-nixen

release-cross: add nix and nixUnstable

pkgs/top-level/release-cross.nix

···
     buildPackages.binutils = nativePlatforms;
     gmp = nativePlatforms;
     libcCross = nativePlatforms;
+    nix = nativePlatforms;
+    nixUnstable = nativePlatforms;
   };
 
   gnuCommon = lib.recursiveUpdate common {