[jnigen] Add build badge (https://github.com/dart-lang/jnigen/issues/1)

pkgs/jnigen/README.md

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+[![Build Status](https://github.com/dart-lang/jnigen/workflows/Dart%20CI/badge.svg)](https://github.com/dart-lang/jnigen/actions?query=workflow%3A%22Dart+CI%22+branch%3Amain)
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 # Experimental generator for FFI+JNI bindings.
 
 This package will generate JNI code to invoke Java from C, and Dart FFI bindings to invoke this C code.