Revert "change install target to match current release structure"

This reverts D-Programming-Language/dmd#3798, as no argument in favor of the change has appeared so far. @braddr, what was your original motivation for the change?

It's not a huge issue, so if there is a good reason for the change, I don't mind it. It's just that the change broke a CI tool I'm working on, and in general I'd rather not complicate life for everyone and part with wide-spread Linux/*nix conventions without a good reason.