Update build badge to GitHub Actions.

Author: tillig

README.md

@@ -4,7 +4,7 @@
 
 Autofac is an [IoC container](http://martinfowler.com/articles/injection.html) for Microsoft .NET. It manages the dependencies between classes so that **applications stay easy to change as they grow** in size and complexity. This is achieved by treating regular .NET classes as *[components](https://autofac.readthedocs.io/en/latest/glossary.html)*.
 
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