texlive.combine: export packages attribute to help find-tarballs.nix

Can be verified using:

  nix-instantiate --eval --json --strict ./maintainers/scripts/find-tarballs.nix --arg expr '(import ./. {}).texlive.combined.scheme-small' 2>/dev/null | jq '.[].name' | grep -E "r[0-9]+\.tar\.xz"

pkgs/tools/typesetting/tex/texlive/combine.nix

@@ -52,6 +52,9 @@ in (buildEnv {
 
   buildInputs = [ makeWrapper ] ++ pkgList.extraInputs;
 
+  # This is set primarily to help find-tarballs.nix to do its job
+  passthru.packages = pkgList.all;
+
   postBuild = ''
     cd "$out"
     mkdir -p ./bin