Hi, we've had the same issue a couple days ago for early playtests. It's unfortunately not an isolated case of Unity's DLL being flagged as a false positive. If you tell defender to keep the file and unzip, you can scan again and the false positive will disappear. It somehow only does that with the zip, and won't do it if I use a rar instead.
Edit: I re-zipped the build in a .rar format and added it to the list of downloads. Try this if Windows Defender still gives you troubles.