Nixpkgs: Darwin breakage is blocking nixpkgs-unstable and staging-19.09

[FRidh]

Describe the bug
Certain core packages don't build for Darwin, blocking the nixpkgs-unstable channel from advancing.

https://hydra.nixos.org/job/nixpkgs/trunk/unstable#tabs-constituents

cc @NixOS/darwin-maintainers

[domenkozar]

To save time for others, here's a segfault that blocks the channel https://nix-cache.s3.amazonaws.com/log/nyr6298pi9zgnlpchv2g8gcsgkfd77ly-swift-corefoundation.drv

[sylvainf911]

I see the build of nixpkgs-unstable seems ok now. But https://nixos.org/channels/nixpkgs-unstable still redirects to https://releases.nixos.org/nixpkgs/nixpkgs-20.03pre197736.91d5b3f07d2. Is this normal?

[sylvainf911]

It seems the nixpkgs-unstable is now redirecting to a recent build.

[vcunat]

Seems fine now.

[FRidh]

I noticed that already on last staging next cycle the issue was happening again
https://hydra.nixos.org/build/108024951

[LnL7]

I wasn't able to reproduce the problem easily locally so I only verified the condition with false https://github.com/NixOS/nixpkgs/pull/73572/files#diff-5d73b7be42e81b19a11c4b19c688ae96R79. Apparently functions in an if condition in combination with set -e behaves not at all as I was expecting.

Removed ninjaBuildPhase in #75528.

[vcunat]

Now it succeeded. It doesn't seem the retry happened, but I'll hope it works; let's close until we encounter the issue again.