Fix `build_codegen!` not finding `@react-native/codegen` in pnpm setups

`build_codegen!` currently assumes that `@react-native/codegen` gets
installed next to `react-native`. In a pnpm setup, it's found under
`/~/react-native/node_modules/@react-native/codegen` instead.

However, as @dmytrorykun pointed out, we don't actually need to build
it outside of this repository.