workflows/tapioca: add after diff

.github/workflows/tapioca.yml

@@ -43,10 +43,9 @@ jobs:
           bundle exec tapioca sync --exclude json
           bundle exec srb rbi hidden-definitions
 
-          git add sorbet
-
           if ! git diff --no-patch --exit-code HEAD -- sorbet; then
             if ! git ls-remote --exit-code --heads origin tapioca-update >/dev/null; then
+              git add sorbet
               git commit -m "sorbet: update RBI files using Tapioca." -m "Autogenerated by [a scheduled GitHub Action](https://github.com/Homebrew/brew/blob/master/.github/workflows/tapioca.yml)."
               echo "::set-output name=committed::true"
             fi