
Reintroduce the `fetch-ssh-keys` service so that GCE images that work with NixOps can once again be built. Also, reformat the code a bit. The service was removed in 88570538b3b19d60b00bc3905bbaaef17e5a5c94, likely due to a comment saying it should be removed. It was still needed for images to work with NixOps, however, and probably needed to be replaced or rewritten rather than removed.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at https://nixos.org/nixos and in the manual in doc/manual.