nix-ld: enable for conda/pixi FHS compatibility

Conda-forge binaries (pixi-managed Python, pip manylinux wheels, etc.)
expect /lib64/ld-linux-x86-64.so.2 and dynamically link against libc
at hardcoded FHS paths that don't exist on NixOS. Without nix-ld,
`pixi install` fails during PyPI source builds with:

  build dispatch initialization failed: failed to query interpreter
  in instantiated prefix

because pixi can't invoke the prefix's Python to read interpreter
metadata. Enabling nix-ld with stdenv.cc.cc.lib and zlib provides
the FHS-style loader and core libraries these tools assume.

hosts/whitegrizzly/default.nix

@@ -15,6 +15,11 @@
   networking.networkmanager.enable = true;
 
   programs.zsh.enable = true;
+  programs.nix-ld.enable = true;
+  programs.nix-ld.libraries = with pkgs; [
+    stdenv.cc.cc.lib
+    zlib
+  ];
 
   users.users.alex = {
     isNormalUser = true;