Postgraphile uses incorrect client for mutations
### Summary

Postgraphile (using graphile pro) does not switch to using the Writer when performing a mutation inside a query resolver.

### Steps to reproduce

- Set `readOnlyConnection` option
- Extend the schema using makeExtendSchemaPlugin, create a new `query`
- Inside the resolver, perform a mutation using the schema and the `graphql` function from build
- Call this query

### Expected results

I'd expect the Postgraphile to know that now inside the resolver we are performing a mutation, therefore it should switch to the Writer PG Client.

### Actual results

The request is failing with errors like "permission denied to insert"

### Additional context

- Node: v12.18.0
- PostgreSQL: 10.13
- @graphile/pro: 1.0.3
- Postgraphile: 4.11.0
