False positive. You do not need a third-party anti-virus tool built by people that did not build macOS and its read-only System partition and built-in security features. That /private/var location is on a read-only partition that even you do not have write permission into, so nothing got installed. Avast is pulling your leg as it has for so many others over the years.
Remove Avast using its vendor's full removal process, and let macOS take care of itself. You won't need to worry about false positives again, and the operating system and other applications will operate as intended.