Bebop: Disable password login for openssh

It is never recommended to enable password authentication for ssh.

Although bebop is intended to always be accessed from local network, it is
indeed exposed to wider internet, and I don't want surprises here.

Also NixOS made declarative SSH addition and deployment so much easier and
friction-free that I don't even want this anymore.

For rare occasions where system breaks or something, I can just login the hard
way, via physical keyboard and disaply. Builds better habits too.

hosts/bebop/configuration.nix

@@ -46,6 +46,7 @@ in
   services.openssh = {
     enable = true;
     permitRootLogin = "prohibit-password";
+    passwordAuthentication = false;
   };
 
   # Make sure dumb ssh-add is run on every interactive entry