darwin.builder: prefer shutting down over halting VM (#208450)

This is preferable because it prevents things like disk corruption (requiring the user to delete the disk image when starting up) that I consistently ran into.

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

@@ -39,7 +39,8 @@ nixos login:
 ```
 
 > Note: When you need to stop the VM, type `Ctrl`-`a` + `c` to open the `qemu`
-> prompt and then type `quit` followed by `Enter`
+> 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`)
 
 To delegate builds to the remote builder, add the following options to your
 `nix.conf` file:

nixos/modules/profiles/macos-builder.nix

@@ -9,7 +9,8 @@ let
 
 in
 
-{ imports = [
+{
+  imports = [
     ../virtualisation/qemu-vm.nix
 
     # Avoid a dependency on stateVersion
@@ -18,8 +19,7 @@ in
         ../virtualisation/nixos-containers.nix
         ../services/x11/desktop-managers/xterm.nix
       ];
-      config = {
-      };
+      config = { };
       options.boot.isContainer = lib.mkOption { default = false; internal = true; };
     }
   ];
@@ -98,11 +98,11 @@ in
       '';
 
     in
-      script.overrideAttrs (old: {
-        meta = (old.meta or { }) // {
-          platforms = lib.platforms.darwin;
-        };
-      });
+    script.overrideAttrs (old: {
+      meta = (old.meta or { }) // {
+        platforms = lib.platforms.darwin;
+      };
+    });
 
   system = {
     # To prevent gratuitous rebuilds on each change to Nixpkgs
@@ -118,10 +118,22 @@ in
     '');
   };
 
-  users.users."${user}"= {
+  users.users."${user}" = {
     isNormalUser = true;
   };
 
+  security.polkit.enable = true;
+
+  security.polkit.extraConfig = ''
+    polkit.addRule(function(action, subject) {
+      if (action.id === "org.freedesktop.login1.power-off" && subject.user === "${user}") {
+        return "yes";
+      } else {
+        return "no";
+      }
+    })
+  '';
+
   virtualisation = {
     diskSize = 20 * 1024;