Use different email address for commits in different repos

[RalfJung]

With tools using subtree/josh-based syncing, we now regularly have commits form one repo be moved to another. This makes it impossible to tell if a given bors commit e.g. in the main rust repo was actually created by the bors of that repo, or if it's some bors commit ported over from a different repo. That then leads to problems like rust-lang/rust#101907 where the various heuristics bootstrap has, which are based on identifying bors commits, all fall apart.

So we need some way to make bors use a different email address for different repos, so that we can tell these commits apart.