That may make sense, I can imagine the way Avast scans and locks the file to try and protect you could still allow the file to be read and executed once installed. It sounds however like the way Steam applies the updates means Avast sees the file being accessed and starts to scan it, while it's being scanned Steam attempts to write the new version of the file and then you get the file locked error.
You could probably add an exclusion path in Avast to tell it not to scan files in that folder although I appreciate this isn't the ideal solution since it slightly reduces your protection against virus infection.
We are looking at whether we can get our files whitelisted by companies like Avast, however I think the turnaround time for us to submit each update might mean that we'd be updating the game before they've even whitelisted the previous version, also I don't know if that would even fix the issue.
I'll also have a look around for other potential solutions, it feels like other games would be affected so maybe there's something else they're doing which we can copy that helps to avoid the issue.