removing support for x86_64-darwin

[zowoq]

https://nixos.org/manual/nixpkgs/unstable/release-notes#x86_64-darwin-26.05
This will be the last release of Nixpkgs to support x86_64-darwin. Platform support will be maintained and binaries built until Nixpkgs 26.05 goes out of support at the end of 2026. For 26.11, due to Apple’s deprecation of the platform and limited build infrastructure and developer time, we will no longer build packages for x86_64-darwin or support building them from source.

As nixpkgs is removing support for x86_64-darwin on unstable after 26.05 is branched we're planning to remove x86_64-darwin support on the nix-community darwin machines, the build box and the CI runner. Unfortunately we don't have the option of leaving x86_64-darwin enabled for stable and disabled for unstable.