Purge prefix path before starting a build.
Previously, the prefix path was left alone, and the build-root was purged if MONO_VERSION changed.
Now the prefix path is *always* removed, as it can contain a partially successful build that is invalid despite MONO_VERSION remaining the same.
This also means that we can't detect MONO_VERSION changes between bockbuild runs on the same host, but purging build-root does little to ensure build consistency compared to actually purging the install path.