Detect and print instructions when app is not building with Hermes patched

[kraenhansen]

For now, the easiest would probably be to read the REACT_NATIVE_OVERRIDE_HERMES_DIR environment variable, from our host package's build.gradle file.

If not set, we should fail the build and print instructions to enable building Hermes from source and setting the variable:

https://reactnative.dev/contributing/how-to-build-from-source#update-your-project-to-build-from-source (I suspect only the Hermes specific substitutions are needed).

export REACT_NATIVE_OVERRIDE_HERMES_DIR = `npx react-native-node-api-modules patch-hermes`