Fix the mysql url

Diesel uses libmysqlclient internally, which tries to be clever by using
a unix socket instead of a network socket if it
sees a database url that contains `localhost` as a host name. That won't
work in this case, as the db is running in a docker container. This
change switches to use `127.0.0.1` as host instead which forces the
usage of a network socket.

.github/workflows/rust.yml

@@ -12,7 +12,7 @@ env:
   REDIS_URL: redis://localhost:6379/1
   MONGODB_URL: mongodb://localhost:27017
   POSTGRES_URL: postgres://postgres:postgres@localhost:5432
-  MYSQL_URL: mysql://root@localhost:3306/public
+  MYSQL_URL: mysql://root@127.0.0.1:3306/public
 
 jobs:
   check: