Remove /etc/nix/id_rsa from example

[jakobkukla]

I couldn't find any mention of this file except sources related with this repository. Furthermore the example changes the permissions of the /etc/nix directory. This breaks the new nix command for non-root users because the nix config file isn't readable anymore.

[talyz]

The purpose of this file is just to serve as an example of how to use the options, not as a default config. It's the ssh key I use for remote building in my setup. I'd prefer to replace this example with something else if it's confusing.

[jakobkukla]

I was aware that this only served as a usage example. Still it's rather confusing since it's next to sensible defaults like /etc/NetworkManager/system-connections and /etc/machine-id. How about we use something like /etc/ssh/ssh_known_hosts instead, to avoid that users who blindly copy the example end up with a broken nix command?

[talyz]

AFAICT, the global known hosts file would mainly be used for declared known hosts and shouldn't need to be persisted, so it wouldn't be a good example either. Maybe we could use the ssh host keys as examples?

[jakobkukla]

Closing as #151 fixed this.