init: run update check in the same zsh process (#9019)

oh-my-zsh.sh

@@ -5,9 +5,7 @@ if [[ -z "$ZSH_CACHE_DIR" ]]; then
 fi
 
 # Check for updates on initial load...
-if [ "$DISABLE_AUTO_UPDATE" != "true" ]; then
-  env ZSH=$ZSH ZSH_CACHE_DIR=$ZSH_CACHE_DIR DISABLE_UPDATE_PROMPT=$DISABLE_UPDATE_PROMPT zsh -f $ZSH/tools/check_for_upgrade.sh
-fi
+source $ZSH/tools/check_for_upgrade.sh
 
 # Initializes Oh My Zsh