nixos/roundcube: fix wrong error log location (#465979)

    services.phpfpm.pools.roundcube = {

      user = if localDB then user else "nginx";

      phpOptions = ''

        error_log = 'stderr'

        error_log = '/dev/stderr'

        log_errors = on

        post_max_size = ${cfg.maxAttachmentSize}

        upload_max_filesize = ${cfg.maxAttachmentSize}