fix: set 'buftype' to nofile instead of nowrite

[loqusion]

For context, I have deactivate set to run UndotreeHide on exit in my lazy.nvim config. When this happens, and I restore the session that was automatically created with :mks, the window created by undotree still persists (ignore the strange window width as that's caused by a separate plugin):

However, if I set 'buftype' to nofile before exiting, it seems to fix the problem:

Setting 'buftype' to nofile also fixes the issue where the buffer title changes when you :cd, although this is minor.