Fix lunch choices w/zsh

zsh doesn't split variables (by default), but it will split command
output. bash will split both. Apparently a portable way to do this in
both is to just turn it into another command: "$(echo $var)".

Test: source build/envsetup.sh; lunch (see list; repeat in bash and zsh)
Change-Id: I9716a8d77aefdf05da8923deb9e37e0b6132b549