Build deltachat-rpc-server for macOS with Nix

Current status on 2025-10-20 can be seen from what is enabled in CI
https://github.com/chatmail/core/blob/2f34a740c7538900dfa04d1c05bcc3775fcfdd6c/.github/workflows/nix.yml#L96-L102

Native build works, cross-compilation from aarch64 to x86-64 works and cross-compilation to aarch64 (`nix build .#deltachat-rpc-server-aarch64-darwin`) still fails because of upstream issue https://github.com/NixOS/nixpkgs/issues/413910

As of 2026-05-05, there is some progress going on upstream, waiting for https://github.com/NixOS/nixpkgs/pull/510292

According to https://github.com/actions/runner-images/ current `macos-latest` runner is an arm64 runner, so we should be able to use `macos-latest` to do cross-build of x86-64 and native build of aarch64 binaries in Nix. I would prefer to do explicit cross-builds even from aarch64 to aarch64 like we do for Linux, but since they are not working native build is the best we can do at the moment.