set rspamd worker type depending on NixOS release
The worker type "proxy" was renamed to "rspamd_proxy" in https://github.com/NixOS/nixpkgs/pull/49809.
See also #138 (comment 116321753).
The worker type "proxy" was renamed to "rspamd_proxy" in https://github.com/NixOS/nixpkgs/pull/49809.
See also #138 (comment 116321753).