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

[Lokito23]

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.

[NotAShelf]

I think this is best addressed after #92.

[Lokito23]

I think this is best addressed after #92.

True, adding support for other priviledge escalation programs is important. After that, the implementation to fix this issue could be discussed.

[athyfr]

This also applies when a new dependency flake is added.