Return target dep as flat list instead of nested list for darwin std

[jost-s]

It came out of debugging a call to overrideSDK on a x86_64-darwin system on the Nix discourse, that the rust-overlay returns a nested list with libiconv as the only element. That caused the override to fail.

lib.optional takes any value and puts it into a list when the condition is met. In this case the value is wrapped in a list, so the result is a nested list.

nix-repl> legacyPackages.aarch64-darwin.lib.optional true ""                                       
[ "" ]

nix-repl> legacyPackages.aarch64-darwin.lib.optional true [""]
[ [ ... ] ]

I suggest to return a flat list instead, as is done a few lines up in the same file.

[jost-s]

overrideSDK will be patched to handle the special case of a nested list anyway. https://github.com/NixOS/nixpkgs/pull/314167

[oxalica]

Thanks for the catch!