.circleci: run 'latest' scenario only if available

.circleci/config.yml

@@ -36,7 +36,7 @@ jobs:
            echo 'No alternative test'
          fi
       - run: |
-         if [[ -z "${CIRCLE_PULL_REQUEST}" ]]; then
+         if [[ -z "${CIRCLE_PULL_REQUEST}" ]] && [[ -d 'molecule/latest' ]]; then
            molecule test -s latest --destroy never
          else
            echo 'Not running latest on PR'