🔧 add yarn prettier install

.circleci/config.yml

@@ -14,7 +14,7 @@ aliases:
       command: yarn install
   check_formatting: &check_formatting
     run:
-      command: yarn format:write  && yarn format:check
+      command: yarn add --dev --exact prettier && yarn format:write  && yarn format:check
   save_cache: &save_cache
     save_cache:
       key: yarn-cache-netlify-{{ checksum "yarn.lock" }}