> I think "cannot be copied" is an exaggeration

It seems that the purpose is to allow local modifications to Modules/Setup. Otherwise, I don't know why the Makefile would print an elaborate message when the file exists.
If this possibility is never used then, true, we can remove the elaborate message and overwrite the existing file instead.

> Not sure why this had high priority - if anybody wants to fix it,
> please go ahead.

How do we fix it? I don't think we have access to the sequence of operations executed by buildbots.

It is high priority because this caused a mess on the buildbots and forced us to revert some perfectly good checkins. I hesitated between high and critical, because infrastructure bugs like this are actively detrimental to development activites.