nixos/roundcube: add tmp directory

nixos/modules/services/mail/roundcube.nix

@@ -132,6 +132,8 @@ in
       $config['plugins'] = [${concatMapStringsSep "," (p: "'${p}'") cfg.plugins}];
       $config['des_key'] = file_get_contents('/var/lib/roundcube/des_key');
       $config['mime_types'] = '${pkgs.nginx}/conf/mime.types';
+      # Roundcube uses PHP-FPM which has `PrivateTmp = true;`
+      $config['temp_dir'] = '/tmp';
       $config['enable_spellcheck'] = ${if cfg.dicts == [] then "false" else "true"};
       # by default, spellchecking uses a third-party cloud services
       $config['spellcheck_engine'] = 'pspell';