BUG: Schema option is not reflected

The schema `schema: 'xxx'` is not reflected when used with `ENV` option inside the `database.json` file.

```json
{
"dev": {
"ENV": "POSTGRES_CONNECTION_STRING",
"schema": "my_schema"
}
}
```

Here is the db-migrate/pg log:
```
[INFO] require: db-migrate-pg
[INFO] connecting
[INFO] connected
[SQL] show server_version_num
[SQL] SHOW search_path
[SQL] SET search_path TO "$user","public"
[SQL] SELECT table_name FROM information_schema.tables WHERE table_name = 'migrations' AND table_schema = 'public'
[INFO] creating table: migrations
[SQL] CREATE TABLE IF NOT EXISTS "migrations" ("id" SERIAL PRIMARY KEY NOT NULL, "name" VARCHAR (255) NOT NULL, "run_on" TIMESTAMP NOT NULL) << schema not reflected
```

---
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