Ensure interface subtypes are added to schema #1434

Job	Run time
@dataplan/pg tests / build (12, 20.x)	3m 30s
PostGraphile mutation tests / build (12, 20.x)	3m 33s
PostGraphile mutation tests (exported as graphql-js) / build (12, 20.x)	9m 52s
PostGraphile mutation tests (exported as typeDefs) / build (12, 20.x)	8m 2s
PostGraphile polymorphic query tests / build (12, 20.x)	3m 23s
PostGraphile polymorphic query tests (exported as graphql-js) / build (12, 20.x)	7m 59s
PostGraphile polymorphic query tests (exported as typeDefs) / build (12, 20.x)	6m 28s
PostGraphile v4 queries tests / build (12, 20.x)	4m 12s
PostGraphile v4 queries tests (exported as graphql-js) / build (12, 20.x)	16m 34s
PostGraphile v4 queries tests (exported as typeDefs) / build (12, 20.x)	12m 45s
Small package tests / build (12, 20.x)	2m 59s
PostGraphile schema tests / build (12, 20.x)	6m 38s
	1h 25m 55s

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