ci: fix mysql myisam test error (#2016)

Running tests with MySQL 8 to avoid the following error with Myisam:

Statement violates GTID consistency: Updates to non-transactional tables can only be done in either autocommitted statements or single-statement transactions, and never in the same statement as updates to transactional tables.

Author: waketzheng

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
           POSTGRES_USER: postgres
         options: --health-cmd=pg_isready --health-interval 10s --health-timeout 5s --health-retries 5
       mysql:
-        image: mysql
+        image: mysql:8
         ports:
           - 3306:3306
         env: