GitHub CI does not support clang less than 9 for Ubuntu

[ekrich]

As of March 29th clang-format 6 is not found for Ubuntu 16.04, 18.04, and 20.04.

If we want to continue to support clang 6 as the minimum supported version we must install it in the CI scripts.

actions/runner-images#2950

[ekrich]

I wonder if we could put something in to cancel runs - I can't believe there is not a button to kill a run.
https://github.com/marketplace/actions/cancel-workflow-action

[ekrich]

As long as all the tests that use clang are using the Docker containers, they are using clang 6. If this is the case then PR will close this issue. #2240

[WojciechMazur]

Fixed in #2240