nix-darwin: sudo --set-home for multiple user activation (#2857)

Changing from `sudo -i` to `sudo -s` messes up activation when multiple
users are managed. `--set-home` should have similar behavior to `-i` in
that the activation script is run from the user's home directory.

Fixes #2856

nix-darwin/default.nix

@@ -134,7 +134,7 @@ in
     system.activationScripts.postActivation.text =
       concatStringsSep "\n" (mapAttrsToList (username: usercfg: ''
         echo Activating home-manager configuration for ${username}
-        sudo -u ${username} -s ${pkgs.writeShellScript "activation-${username}" ''
+        sudo -u ${username} -s --set-home ${pkgs.writeShellScript "activation-${username}" ''
           ${lib.optionalString (cfg.backupFileExtension != null)
             "export HOME_MANAGER_BACKUP_EXT=${lib.escapeShellArg cfg.backupFileExtension}"}
           ${lib.optionalString cfg.verbose "export VERBOSE=1"}