message: add GitHub Action badge
diff: # RustyHermit - A Rust-based, lightweight unikernel
  [![Build Status](https://git.rwth-aachen.de/acs/public/hermitcore/libhermit-rs/badges/master/pipeline.svg)](https://git.rwth-aachen.de/acs/public/hermitcore/libhermit-rs/pipelines)
  -[![Build Status](https://travis-ci.org/hermitcore/libhermit-rs.svg?branch=master)](https://travis-ci.org/hermitcore/libhermit-rs)
  +![Actions Status](https://github.com/hermitcore/libhermit-rs/workflows/Build/badge.svg)
  [![License](https://img.shields.io/crates/l/rusty-hermit.svg)](https://img.shields.io/crates/l/rusty-hermit.svg)
  [![Slack Status](https://radiant-ridge-95061.herokuapp.com/badge.svg)](https://radiant-ridge-95061.herokuapp.com)
  