yabai: disable parallel building

Upstream's Makefile declares clean-build and bin/yabai as independent
prerequisites of all/install. With parallel make, clean-build can remove
./bin while clang is linking bin/yabai, causing ld to fail with errno=2.

This was hit by Hydra and nix-darwin with -j2/-j3. Upstream's Homebrew
HEAD formula also builds from source with make -j1 install, so keep
nixpkgs building from source and serialize make.

pkgs/by-name/ya/yabai/package.nix

@@ -34,7 +34,8 @@ stdenv.mkDerivation (finalAttrs: {
     apple-sdk_15
   ];
 
-  enableParallelBuilding = true;
+  # Upstream Makefile races clean-build against linking under parallel make.
+  enableParallelBuilding = false;
 
   installPhase = ''
     runHook preInstall