nixos: allow files to be symlinked anywhere

[lovesegfault]

This removes the current restriction on files needing to exist in /etc and allows them to be symlinked anywhere on disk.

[lovesegfault]

@talyz review ping :)

[talyz]

@lovesegfault Sorry I haven't reviewed this earlier; I believe we discussed this approach in another issue, so I thought I already had.

My issue with this is that it doesn't clean up symlinks when you switch to a new generation. This is something that we get for free with environment.etc, but we should also be able to use systemd units, as we do currently with the the home-manager bind mounts. That would also let us dynamically choose whether we symlink or bind mount files (symlink if the real file doesn't exist, bind mount if it does).