Dynamically generate `matrix` entry in GH Actions workflow

[jsf9k]

💡 Summary

It should be possible to dynamically generate the matrix entry in the test portion of the build workflow.

Motivation and context

This would reduce the burden on the repo maintainer.

Implementation notes

• Here is an example of a workflow that dynamically generates the matrix entry. This idea is briefly discussed here.
• We can parse the list of scenarios by listing the directories under molecule.
• We can parse the platforms and architectures for each scenario by parsing its molecule.yml file.
• In most cases the platforms and architectures will the same for each scenario, but differences can be handled by taking the superset and adding additional exclude configurations to the matrix.

[mcdonnnj]

Just a note but my testing of dynamic generation of the matrix entry resulted in very unhappy linters. This was a while ago so it may be better now but I wanted to mention it.