nixos/ec2-data: sshd.service -> sshd-keygen.service

SSH key generation was split out into its own systemd service in
https://github.com/NixOS/nixpkgs/pull/372979, but dependent service
definitions weren't updated.

The `apply-ec2-data` service needs to run before SSH key generation,
as it fetches host keys defined in ec2 user data and these keys should
take priority over generating new ones. Currently, the ordering
doesn't specify which should run first of `apply-ec2-data` and
`sshd-keygen`; in practice it seems that `sshd-keygen` often wins the
race, though.

Update the dependencies so that `apply-ec2-data` always runs first.

nixos/modules/virtualisation/ec2-data.nix

@@ -23,9 +23,9 @@ with lib;
 
       wantedBy = [
         "multi-user.target"
-        "sshd.service"
+        "sshd-keygen.service"
       ];
-      before = [ "sshd.service" ];
+      before = [ "sshd-keygen.service" ];
       after = [ "fetch-ec2-metadata.service" ];
 
       path = [ pkgs.iproute2 ];
@@ -80,7 +80,7 @@ with lib;
     systemd.services.print-host-key = {
       description = "Print SSH Host Key";
       wantedBy = [ "multi-user.target" ];
-      after = [ "sshd.service" ];
+      after = [ "sshd-keygen.service" ];
       script = ''
         # Print the host public key on the console so that the user
         # can obtain it securely by parsing the output of