d5fb47bcc8
Add the two *-apple-darwin targets to rust-toolchain.toml and apple-sdk + libiconv to the flake devShell (darwin only) so the nix toolchain links AppKit; make cargo-zigbuild/zig Linux-only (macOS builds natively). Repoint the throwaway macOS workflow to actually build both darwin targets through the flake on the Tart runner — the first real check of the macOS leg, which can't be verified locally. Delete once release-macos lands.