darwin.builder: auto-login as the builder user (#208772)

… as suggested by @NiklasGollenstede in:

https://github.com/NixOS/nixpkgs/pull/206951#issuecomment-1369020601

This simplifies the user experience for logging into and
debugging the machine and also simplifies the instructions for
shutting down the machine gracefully.

doc/builders/special/darwin-builder.section.md

@@ -28,19 +28,21 @@ Password:
 ```
 
 … so that it can install a private key used to `ssh` into the build server.
-After that the script will launch the virtual machine:
+After that the script will launch the virtual machine and automatically log you
+in as the `builder` user:
 
 ```
 <<< Welcome to NixOS 22.11.20220901.1bd8d11 (aarch64) - ttyAMA0 >>>
 
 Run 'nixos-help' for the NixOS manual.
 
-nixos login:
+nixos login: builder (automatic login)
+
+
+[builder@nixos:~]$
 ```
 
-> Note: When you need to stop the VM, type `Ctrl`-`a` + `c` to open the `qemu`
-> prompt and then type `system_powerdown` followed by `Enter`, or run `shutdown now`
-> as the `builder` user (e.g. `ssh -i keys/builder_ed25519 builder@localhost shutdown now`)
+> Note: When you need to stop the VM, run `shutdown now` as the `builder` user.
 
 To delegate builds to the remote builder, add the following options to your
 `nix.conf` file:

nixos/modules/profiles/macos-builder.nix

@@ -59,10 +59,14 @@ in
     trusted-users = [ "root" user ];
   };
 
-  services.openssh = {
-    enable = true;
+  services = {
+    getty.autologinUser = user;
 
-    authorizedKeysFiles = [ "${keysDirectory}/%u_${keyType}.pub" ];
+    openssh = {
+      enable = true;
+
+      authorizedKeysFiles = [ "${keysDirectory}/%u_${keyType}.pub" ];
+    };
   };
 
   system.build.macos-builder-installer =