nix: use -ld_new on macOS

The new parallel macOS linker reduces link time for the debug `jj` binary from
3s to 0.7s on my M2 Macbook Air, which is a significant reduction for nearly
no cost at all.

However, enabling it is rather strange because we have to give a full path to
the /Applications directory, and I don't know of a good way to encapsulate it
with existing Nix primitives inside Nixpkgs...

This change requires Sonoma and Xcode 15, but in theory I think we could target
a lower macOS SDK version in order to produce binaries that are more backwards
compatible, so the only real cost is that developers who also use Nix would
require Sonoma.

flake.nix

@@ -177,9 +177,11 @@
           export LIBSSH2_SYS_USE_PKG_CONFIG=1
 
           export RUSTFLAGS="-Zthreads=0"
-        '' + pkgs.lib.optionalString useMoldLinker ''
+        '' + (if (useMoldLinker) then ''
           export RUSTFLAGS+=" -C link-arg=-fuse-ld=mold -C link-arg=-Wl,--compress-debug-sections=zstd"
-        '' + darwinNextestHack;
+        '' else ''
+          export RUSTFLAGS+=" -C link-arg=-fuse-ld=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld -C link-arg=-ld_new"
+        '') + darwinNextestHack;
       };
     }));
 }