nixos/gdm: update gdm-greeter user homes

This fixes gdm not being able to save config changes made on the log-in
screen, such as turning on accessibility options. The new values for the
`gdm-greeter{-2,-3,-4,-5}` users map onto what userdbd would create for us.
We need writable home directories to interact with `dbus`, as well as to
populate the `/run/gdm/home/gdm-greeter/.config/environment.d/` files.

`gnome-inital-setup-done` is now seemingly called `gdm.ran-initial-setup`:
https://gitlab.gnome.org/GNOME/gdm/-/blob/gnome-50/daemon/gdm-manager.c?ref_type=heads#L64

nixos/modules/services/display-managers/gdm.nix

@@ -44,6 +44,20 @@ let
   defaultSessionName = config.services.displayManager.defaultSession;
 
   setSessionScript = pkgs.callPackage ../x11/display-managers/account-service-util.nix { };
+
+  greeterUsers = lib.genAttrs' [ null 1 2 3 4 ] (
+    i:
+    let
+      # adding 1 to create `gdm-greeter{-2,-3,-4,-5}`
+      suffix = lib.optionalString (i != null) "-${toString (i + 1)}";
+    in
+    lib.nameValuePair "gdm-greeter${suffix}" {
+      isSystemUser = true;
+      uid = 60578 + (if i == null then 0 else i);
+      group = "gdm";
+      home = "/run/gdm/home/gdm-greeter${suffix}";
+    }
+  );
 in
 
 {
@@ -196,24 +210,8 @@ in
           group = "gdm";
           description = "GDM user";
         };
-
-        gdm-greeter = {
-          isSystemUser = true;
-          uid = 60578;
-          group = "gdm";
-          home = "/run/gdm";
-        };
       }
-
-      (lib.genAttrs' [ 1 2 3 4 ] (
-        i:
-        lib.nameValuePair "gdm-greeter-${toString i}" {
-          isSystemUser = true;
-          uid = 60578 + i;
-          group = "gdm";
-          home = "/run/gdm-${toString i}";
-        }
-      ))
+      greeterUsers
     ];
 
     users.groups.gdm.gid = config.ids.gids.gdm;
@@ -255,17 +253,20 @@ in
       };
     };
 
-    systemd.tmpfiles.rules = [
-      "d /run/gdm/.config 0711 gdm gdm"
-    ]
-    ++ lib.optionals config.services.pulseaudio.enable [
-      "d /run/gdm/.config/pulse 0711 gdm gdm"
-      "L+ /run/gdm/.config/pulse/${pulseConfig.name} - - - - ${pulseConfig}"
-    ]
-    ++ lib.optionals config.services.gnome.gnome-initial-setup.enable [
-      # Create stamp file for gnome-initial-setup to prevent it starting in GDM.
-      "f /run/gdm/.config/gnome-initial-setup-done 0711 gdm gdm - yes"
-    ];
+    systemd.tmpfiles.rules =
+      lib.optionals config.services.pulseaudio.enable (
+        lib.concatLists (
+          lib.mapAttrsToList (name: user: [
+            "d ${user.home}/.config 0711 ${name} gdm"
+            "d ${user.home}/.config/pulse 0711 ${name} gdm"
+            "L+ ${user.home}/.config/pulse/${pulseConfig.name} - - - - ${pulseConfig}"
+          ]) greeterUsers
+        )
+      )
+      ++ lib.optionals config.services.gnome.gnome-initial-setup.enable [
+        # Create stamp file for gnome-initial-setup to prevent it starting in GDM.
+        "f /run/gdm/gdm.ran-initial-setup 0711 gdm gdm - yes"
+      ];
 
     # Otherwise GDM will not be able to start correctly and display Wayland sessions
     systemd.packages = [