feat(nix): move ollama to `nix-darwin`

Signed-off-by: Seongmin Lee <[e-mail]>

nix/darwin.nix

···
       pkgs.tmux
       pkgs.tree
       pkgs.vim
+      pkgs.ollama
       # I don't want to touch this due to current neovim settings.
       # remove it when lux.nvim is out
       (pkgs-unstable.lua5_1.withPackages (ps: with ps; [luarocks]))

nix/flake.nix

···
         pkgs.jpm
         pkgs.meld
         pkgs.nodejs_24
-        pkgs.ollama
         pkgs.qmk
         pkgs.ripgrep
         pkgs.sumneko-lua-language-server