schema.graphql export is not awaited in getGraphQLSchema
https://github.com/graphile/postgraphile/blob/4164d437e57a30ff00ecbb4675943a0924c54ec9/src/postgraphile/postgraphile.ts#L159

the above is not awaited which leads to my issue

I'm trying to export schema.graphql in CI and can't use CLI to replicate complex library usage with plugins.

```ts
import { OPTIONS } from 'db/graphql/postgraphile';
config();

(async () => {
  const pgPool = new Pool({ connectionString: process.env.DATABASE_URL });
  const { getGraphQLSchema } = getPostgraphileSchemaBuilder(
    pgPool,
    ['schema'],
    { ...OPTIONS }
  );
  await getGraphQLSchema();
  await setTimeout(1000);  // I'd like to remove this!
  exit();
})();
```

I think the resolution may be awaiting the export?