The build.log file is quite annoying because it is placed in the invoked project's source tree by default. It is not configurable, except to disable it entirely with the noLog option.
This causes issues, such as what is identified in
RAT-160, where certain plugins behave in unexpectedly bad ways when it appears in the invoked project's root.
Ideally, I'd love for the build.log file to be created outside of the invoked project's source tree entirely. The most sensible, I think, is in the invoking project's target/ directory, named uniquely for that invocation. Another option is in the invoked project's target directory (which may not otherwise exist, so this may not be suitable). A third option is to make the location completely configurable, and leave it up to the user to decide where it goes.
The important thing to remember is that not every plugin on an executed project can be configured to ignore this file, so it definitely shouldn't be created in the invoked project's root by default when there isn't an option to put it elsewhere.