add rustc and cargo to the cross trunk

Compiling them can take hours, and with rust specifically it's quite common to
cross compile (e.g. to musl/static for easier distribution)

pkgs/top-level/release-cross.nix

@@ -34,6 +34,8 @@ let
     nix = nativePlatforms;
     nixUnstable = nativePlatforms;
     mesa = nativePlatforms;
+    rustc = nativePlatforms;
+    cargo = nativePlatforms;
   };
 
   gnuCommon = lib.recursiveUpdate common {