(I commented this before but it apparently went poof).

We don't really want to leave files from an old build hanging, so erasing this file only when building a personalized build might not be a good idea. On the other hand, erasing every time won't cut it either because that will be problematic for devs wanting to test this just by manually adding the file to the dir. 
So probably a better solution (maybe for a followup bug) would be adding the file to the zip directly instead of polluting the work directory. That way if the dev has manually added a test file to that dir we don't delete it and if VARIANT_PATH is defined then we just overwrite it on the zip with the GAIA_DISTRIBUTION_DIR file without touching his file.