chore: add the --ephemeral server flag (#16126)

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.

cli/server_createadminuser.go

@@ -54,7 +54,7 @@ func (r *RootCmd) newCreateAdminUserCommand() *serpent.Command {
 
 			if newUserDBURL == "" {
 				cliui.Infof(inv.Stdout, "Using built-in PostgreSQL (%s)", cfg.PostgresPath())
-				url, closePg, err := startBuiltinPostgres(ctx, cfg, logger)
+				url, closePg, err := startBuiltinPostgres(ctx, cfg, logger, "")
 				if err != nil {
 					return err
 				}