Cancel upgrade if $ZSH is not writable

from #2360

tools/check_for_upgrade.sh

@@ -20,6 +20,12 @@ if [[ -z "$epoch_target" ]]; then
   epoch_target=13
 fi
 
+[ -f ~/.profile ] && source ~/.profile
+
+# Cancel upgrade if the current user doesn't have write permissions for the
+# oh-my-zsh directory.
+[[ -w "$ZSH" ]] || return 0
+
 if [ -f ~/.zsh-update ]
 then
   . ~/.zsh-update