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, 6 insertions, 2 deletions

diff --git a/package.nix b/package.nix
index f65611ea6..ef6f317e8 100644
--- a/package.nix
+++ b/package.nix
@@ -376,8 +376,6 @@ stdenv.mkDerivation (finalAttrs: {
     platforms = lib.platforms.unix;
   };
 
-  passthru.perl-bindings = pkgs.callPackage ./perl { inherit fileset stdenv; };
-
   # Export the patched version of boehmgc.
   # flake.nix exports that into its overlay.
   passthru = {
@@ -509,5 +507,11 @@ stdenv.mkDerivation (finalAttrs: {
           '';
         }
       );
+
+    perl-bindings = pkgs.callPackage ./perl { inherit fileset stdenv; };
+
+    binaryTarball = pkgs.callPackage ./nix-support/binary-tarball.nix {
+      nix = finalAttrs.finalPackage;
+    };
   };
 })