Nix build fails on macOS

[aflatter]

On macOS Big Sur (11.4), building with and without flakes (nix run), I get the following error:

    = note: ld: file not found: /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation
            clang-7: error: linker command failed with exit code 1 (use -v to see invocation)

Using niv update nixpkgs -b nixpkgs-unstable, things seem to work alright. I don't know what exactly the problem is, but here's a couple of related information that I've found:

• https://github.com/holochain/holonix/issues/211
• https://discourse.nixos.org/t/big-sur-frameworks-issue/10810/3
• Linking frameworks on macOS fails in sandboxed mode NixOS/nixpkgs#41099

I'll check nixpkgs 21.05 and see if it works.