linux: add openssh and tb user account

system/linux/openssh.nix

@@ -0,0 +1,14 @@
+{ ... }:
+{
+  # This setups a SSH server. Very important if you're setting up a headless system.
+  # Feel free to remove if you don't need it.
+  services.openssh = {
+    enable = true;
+    settings = {
+      # Require a ssh key login for root.
+      PermitRootLogin = "prohibit-password";
+      # Use keys only. Remove if you want to SSH using password (not recommended)
+      PasswordAuthentication = true;
+    };
+  };
+}