This is the base response anti-virus programs give you regarding .exe files from other sources.Tried to download the installer and google has marked it as a infected file and gives me the message
Sorry, this file is infected with a virus.
Only the owner is allowed to download infected files.
A paranoid person would download this into a virtual environment and unpack and use antivirus and testing there.
If its clean (which all players on here claim it is) you could bring this up in the forums for your antivirus provider, that the program is giving you vague or incorrect information.
An alternative is also to instead go to github and just download the code without the .exe:
GitHub - LoneGazebo/Community-Patch-DLL: Community Patch for Civilization V - Brave New World
Community Patch for Civilization V - Brave New World - LoneGazebo/Community-Patch-DLL