The argument `-u` / `--update` should ask for `sudo` if the flake isn't user writable

Currently, the `-u` argument on the `nh os switch` and `nh os boot` options fails to account a flake that isn't user writable, so it attempts to update the inputs, only to fail to write the update due to `flake.lock` not being writable by the user running the command.

It should check if the flake is writable, and if it isn't, it should ask for `sudo` to be able to update the flake.