Specify ssh-keys as list of strings, rather than having a single string.

Older versions of nixpkgs allowed you to specify multiple keys by having newline separated keys in your string. However, this worked essentially by accident, and is now explictly disallowed.

I noticed this because I had configured multiple keys for the local grid. This isn't currently impacting my ability to work, but it seems like a worthwhile improvement anyway. This will be necessary (for example) if/when multiple people are given root access to our storage nodes.

CC: @jcalderone, @florian This will require updating morph/grid/local/public-keys/users.nix locally if/when this lands.