Cannot select a 'via' attribute from PgUpdateSingleStep
### Summary

We've just introduced some complex RLS permissions to part of our API. The policies seem to work correctly, and data is returned correctly, but this error is also thrown. This happens after updating a table row using the generated `update` mutation.

```
{
  error: Error: Cannot select a 'via' attribute from PgUpdateSingleStep
      at PgUpdateSingleStep.get (/node_modules/.pnpm/@dataplan+pg@0.0.1-beta.27_@dataplan+json@0.0.1-beta.25_grafast@0.1.1-beta.13_graphile-_20a976b458ca0b8d27e387178a47f6f2/node_modules/@dataplan/pg/dist/steps/pgUpdateSingle.js:105:19)
      at /node_modules/.pnpm/graphile-build-pg@5.0.0-beta.32_@dataplan+pg@0.0.1-beta.27_@dataplan+json@0.0.1-beta.25_453e922d01adb72b827830983c00968b/node_modules/graphile-build-pg/dist/plugins/PgAttributesPlugin.js:52:36
      at /node_modules/.pnpm/grafast@0.1.1-beta.13_graphile-config@0.0.1-beta.9_graphql@16.9.0_tamedevil@0.0.0-beta.7/node_modules/grafast/dist/engine/OperationPlan.js:1125:235
      at withFieldArgsForArguments (/node_modules/.pnpm/grafast@0.1.1-beta.13_graphile-config@0.0.1-beta.9_graphql@16.9.0_tamedevil@0.0.0-beta.7/node_modules/grafast/dist/operationPlan-input.js:399:20)
      at withGlobalLayerPlan (/node_modules/.pnpm/grafast@0.1.1-beta.13_graphile-config@0.0.1-beta.9_graphql@16.9.0_tamedevil@0.0.0-beta.7/node_modules/grafast/dist/engine/lib/withGlobalLayerPlan.js:12:25)
      at OperationPlan.planField (/node_modules/.pnpm/grafast@0.1.1-beta.13_graphile-config@0.0.1-beta.9_graphql@16.9.0_tamedevil@0.0.0-beta.7/node_modules/grafast/dist/engine/OperationPlan.js:1125:73)
      at OperationPlan.processGroupedFieldSet (/node_modules/.pnpm/grafast@0.1.1-beta.13_graphile-config@0.0.1-beta.9_graphql@16.9.0_tamedevil@0.0.0-beta.7/node_modules/grafast/dist/engine/OperationPlan.js:721:48)
      at OperationPlan.planSelectionSet (/node_modules/.pnpm/grafast@0.1.1-beta.13_graphile-config@0.0.1-beta.9_graphql@16.9.0_tamedevil@0.0.0-beta.7/node_modules/grafast/dist/engine/OperationPlan.js:817:14)
      at OperationPlan.planIntoOutputPlan (/node_modules/.pnpm/grafast@0.1.1-beta.13_graphile-config@0.0.1-beta.9_graphql@16.9.0_tamedevil@0.0.0-beta.7/node_modules/grafast/dist/engine/OperationPlan.js:950:22)
      at OperationPlan.processGroupedFieldSet (/node_modules/.pnpm/grafast@0.1.1-beta.13_graphile-config@0.0.1-beta.9_graphql@16.9.0_tamedevil@0.0.0-beta.7/node_modules/grafast/dist/engine/OperationPlan.js:763:26),
  extensions: undefined
}
```

The table being updated is referenced in a polymorphic table, but the update mutation and the queried result is not using the polymorphic table, it's quering the "child" table directly.