Envsetup: Fix lunch choice with number in zsh

(zsh)$ source build/envsetup.sh; lunch 23
will result in selecting #22 because array in zsh starts from 1
instead of 0. This CL is to fix this issue.

Bug: b/117202855
Test: below commands should have the same output:
    (zsh)$ source build/envsetup.sh; lunch 23
    (zsh)$ source build/envsetup.sh; lunch aosp_walleye-userdebug

Change-Id: I0570585417878bc7c73eda0e1a416232fe147fb4