ci: configure the database to run two-phase-commit tests

ci: configure the database to run two-phase-commit tests

We can't use the Github Actions PostgreSQL service because it doesn't
seem possible to configure the server for stored transactions:

- the service yaml definition doesn't take args to pass to run
- we can't overwrite the entry point with a script of ours because the
  service starts before actions/checkout
- the postgres image doesn't allow to pass extra parameters to the
  server via an env var