[RFC] CMake: Use DESTDIR for helptags generation.

When `DESTDIR` is used (`make DESTDIR=... install`), it is [ignored by the helptags generation](https://aur.archlinux.org/packages/neovim-git) (#1106), so during package building, instead of running the command in e.g. `/home/user/neovim-package/build/usr/local/share/nvim/...` it tries to execute in `/usr/local/share/nvim/...`, which is not what it's supposed to do. Adding `DESTDIR` to the working directory should fix that.

Ping @jszakmeister. Sorry about that :-(