fix nix build on darwin
The libc crate links against System and libiconv on darwin. Libc is included as a transitive dependency: clap -> atty -> libc. Confirmed that the build used to work in da26309f, but I haven't actually bisected for the commit that broke it. I'm guessing that an update to one of the flake dependencies changed the available libraries?
The build failure looks like this:
...
> = note: ld: library not found for -liconv
> clang-11: error: linker command failed with exit code 1 (use -v to see invocation)
>
>
> error: could not compile `cargo_plugin_runner` due to previous error
For full logs, run 'nix log /nix/store/04mkalaa07j2g7dkl0ybwz3rkiqr5m92-cargo_plugin_runner-0.1.0.drv'