ex-vi and vim should conflict

Since both install `bin/ex` and `bin/view`, they should conflict. This
should fix #25657.

Ideally, this is a temporary fix. I would like to find out if it's possible
to have `ex-vi` install `bin/ex` and `bin/view` with a prefix (à la GNU's
`coreutils`). If so, it would be possible to have it all.