ci: pin mysql8 to mysql 8.0.18 (not latest) (#293)

Following my bug report in MySQL issue tracker,
I've pinned MySQL8 version to be 8.0.18 until
we figure out if it's a bug or a new change that
we need to support.

Link: https://bugs.mysql.com/bug.php\?id\=98250

.circleci/config.yml

@@ -67,7 +67,7 @@ jobs:
         environment:
           <<: *mysql_env
           MYSQL_TCP_PORT: 3307
-      - image: circleci/mysql:8
+      - image: circleci/mysql:8.0.18
         environment:
           <<: *mysql_env
           MYSQL_TCP_PORT: 3308