Another PR to address https://github.com/coder/coder/issues/15109.
Changes:
- Introduces the `--ephemeral` flag, which changes the Coder config
directory to a temporary location. The config directory is where the
built-in PostgreSQL stores its data, so using a new one results in a
deployment with a fresh state.
The `--ephemeral` flag is set to replace the `--in-memory` flag once the
in-memory database is removed.