Change default zshrc to export $ZSH (as required by check_for_upgrade.sh).

templates/zshrc.zsh-template

 # Path to your oh-my-zsh configuration.
-ZSH=$HOME/.oh-my-zsh
+export ZSH=$HOME/.oh-my-zsh
 
 # Set name of the theme to load.
 # Look in ~/.oh-my-zsh/themes/