keybase: mark as broken on x86_64-darwin
Error from CI job "OfBorg / keybase, keybase-gui, keybase-gui.passthru.tests, keybase.passthru.tests on x86_64-darwin":
```
> In file included from /nix/store/xxbz3p03wa8wlbhb8c91ndh3f2rad1jp-apple-framework-CoreMedia/Library/Frameworks/CoreMedia.framework/Headers/CMSampleBuffer.h:56:
> In file included from /nix/store/xxbz3p03wa8wlbhb8c91ndh3f2rad1jp-apple-framework-CoreMedia/Library/Frameworks/CoreMedia.framework/Headers/CMFormatDescription.h:27:
> /nix/store/bvqircp5apqaqyk57r8m4z38hfbdabfq-apple-framework-AudioToolbox/Library/Frameworks/AudioToolbox.framework/Headers/AudioFormat.h:161:8: error: redefinition of 'AudioFormatListItem'
> struct AudioFormatListItem
> ^
> /nix/store/a67qmarhk5cca9a4s19r0ys3q99kyndp-apple-framework-CoreAudioTypes-11.0.0/Library/Frameworks/CoreAudioTypes.framework/Headers/CoreAudioBaseTypes.h:1412:8: note: previous definition is here
> struct AudioFormatListItem
> ^
> 1 error generated.
For full logs, run 'nix log /nix/store/2avqqjb3svdzmnqn608vm79z2w9vx8zj-keybase-6.2.4.drv'.
```
From: https://github.com/NixOS/nixpkgs/pull/279435/checks?check_run_id=20351324899
