Fix non-POSIX conditional syntax

Fixes #8416

tools/upgrade.sh

@@ -33,7 +33,7 @@ git config receive.fsck.zeroPaddedFilemode ignore
 
 # Update upstream remote to ohmyzsh org
 remote=$(git remote -v | awk '/https:\/\/github\.com\/robbyrussell\/oh-my-zsh\.git/{ print $1; exit }')
-if [[ -n "$remote" ]]; then
+if [ -n "$remote" ]; then
   git remote set-url "$remote" "https://github.com/ohmyzsh/ohmyzsh.git"
 fi