fix(hc): Add missing using (#54188)

This using is required so that the transaction is opened to the correct
database when running in split-db mode.

Author: mrduncan

tests/sentry/mediators/test_mediator.py

@@ -137,6 +137,8 @@ def call(self):
 
     def test_automatic_transaction(self):
         class TransactionMediator(Mediator):
+            using = router.db_for_write(User)
+
             def call(self):
                 User.objects.create(username="beep")
                 raise RuntimeError()