packaging: Move binaryTarball to a passthru attr in package.nix

This is motivated by flakes being bad and all the stuff that calls
things by "system" being utterly unable to cope with cross compilation.
So if we go shove it in package.nix it is suddenly usable from cross
contexts.

Usage:

```
nix build -L .#nix-riscv64-linux.binaryTarball
```

Change-Id: I702ebf2ac5bd9d1c57662f968b000073134df336

1 files changed, 2 insertions, 6 deletions

diff --git a/flake.nix b/flake.nix
index 8f93127f7..792387da4 100644
--- a/flake.nix
+++ b/flake.nix
@@ -152,8 +152,6 @@
         }
       );
 
-      binaryTarball = nix: pkgs: pkgs.callPackage ./nix-support/binary-tarball.nix { inherit nix; };
-
       overlayFor =
         getStdenv: final: prev:
         let
@@ -236,13 +234,11 @@
         );
 
         # Perl bindings for various platforms.
-        perlBindings = forAllSystems (system: nixpkgsFor.${system}.native.nix.perl-bindings);
+        perlBindings = forAllSystems (system: nixpkgsFor.${system}.native.nix.passthru.perl-bindings);
 
         # Binary tarball for various platforms, containing a Nix store
         # with the closure of 'nix' package.
-        binaryTarball = forAllSystems (
-          system: binaryTarball nixpkgsFor.${system}.native.nix nixpkgsFor.${system}.native
-        );
+        binaryTarball = forAllSystems (system: nixpkgsFor.${system}.native.nix.passthru.binaryTarball);
 
         # docker image with Lix inside
         dockerImage = lib.genAttrs linux64BitSystems (system: self.packages.${system}.dockerImage);