nixos/users-groups: allow changing default home directory

nixos/modules/config/users-groups.nix

@@ -477,7 +477,7 @@ let
         (mkIf config.isNormalUser {
           group = mkDefault "users";
           createHome = mkDefault true;
-          home = mkDefault "/home/${config.name}";
+          home = mkDefault "${cfg.defaultUserHome}/${config.name}";
           homeMode = mkDefault "700";
           useDefaultShell = mkDefault true;
           isSystemUser = mkDefault false;
@@ -753,6 +753,14 @@ in
       '';
     };
 
+    users.defaultUserHome = mkOption {
+      type = types.str;
+      default = "/home";
+      description = ''
+        The default home directory for normal users.
+      '';
+    };
+
     # systemd initrd
     boot.initrd.systemd.users = mkOption {
       description = ''

nixos/modules/programs/shadow.nix

@@ -217,7 +217,7 @@ in
           # /etc/default/useradd: configuration for useradd.
           "default/useradd".source = pkgs.writeText "useradd" ''
             GROUP=100
-            HOME=/home
+            HOME=${config.users.defaultUserHome}
             SHELL=${utils.toShellPath config.users.defaultUserShell}
           '';
         };