Ability to ignore builds from average time calc

not sure how practical this would be, but sometimes my build fails for non build related issues (the box it's on is windows so anything's possible). I know one of my builds takes an average of 120mins, however two builds failed within 10 mins, so the average has dropped to 60 mins. I'd rather it didn't.

clearly the build timer is a bonus anyway, so it's not too much of a problem, but it would be nice