Introduce GitHub Actions workflow to build Ruby with MMTk

[eightbitraptor]

I propose adding a GitHub actions workflow to this fork to build Ruby with MMTk.

Having this will likely expose complexities in the build process that will potentially need to be addressed before we can upstream a patch.

We may be able to build on the work done by The YJIT build workflow to help us achieve this.

[wks]

Good idea. It will also enable us to do binding tests on Ruby when mmtk-core introduces an API change.