deps: send DEPS_PREFIX to cmake
deps: send DEPS_PREFIX to cmake

DEPS_DIR was replaced with DEPS_PREFIX by
https://github.com/neovim/neovim/pull/1588

sschaef/bot-ci,ZyX-I/neovim-bot-ci,fwalch/bot-ci,neovim/bot-ci,ZyX-I/neovim-bot-ci,fwalch/bot-ci,neovim/bot-ci,fwalch/bot-ci,sschaef/bot-ci,sschaef/bot-ci,neovim/bot-ci,ZyX-I/neovim-bot-ci