refactor(deploy-whitby): use nvd instead of nix-diff

nvd only shows us changed versions of packages, as well as
added/removed packages, which means that for the majority of depot
packages nothing will be displayed

however, the current output of nix-diff is not usable anyways, so
having something that can be looked at is better than nothing

Change-Id: Iefbd8139c7ccf5c88ed1209897abdb2ae9302e91
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4868
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

ops/deploy-whitby/default.nix

@@ -16,7 +16,7 @@ pkgs.stdenv.mkDerivation {
         ansi2html
         git
         jq
-        nix-diff
+        nvd
       ]}
   '';