fix(picker.spinner): when parent win is not float win (#2487)

When use checkout PR action from `gh://` buffer

```
vim.schedule callback: snacks.nvim/lua/snacks/picker/util/spinner.lua:103: attempt to perform arithmetic on field 'zindex' (a nil value)
stack traceback:
  snacks.nvim/lua/snacks/picker/util/spinner.lua:103: in function 'loading'
  snacks.nvim/lua/snacks/gh/actions.lua:796: in function '_run'
  snacks.nvim/lua/snacks/gh/actions.lua:791: in function 'fn'
  snacks.nvim/lua/snacks/picker/util/init.lua:90: in function 'on_choice'
  snacks.nvim/lua/snacks/picker/select.lua:59: in function <snacks.nvim/lua/snacks/picker/select.lua:58>
```