Unfortunately, right now the chance of providing a Linux build for Steam seems very unlikely, but we'll try to make it happen so we're not completely discarding the possibility yet. We just don't want to make any promises in that regard in case it doesn't work out in the end.
In any case, the DLC will also be available here on itch.io on a pay-what-you-want basis, and it's definitely the option we would encourage taking because the cut Steam takes from every sale is pretty savage.
Thank you for your comment and the support, we really appreciate it.