Publish coverage report to Checks on GitHub

[heaths]

Part of the work started for #14427, publish the HtmlSummary report to GitHub as a check. Note that this may need to actually be markdown. See https://docs.github.com/en/free-pro-team@latest/rest/reference/checks#update-a-check-run for details.

[heaths]

Azure Pipelines is doing this if code coverage reports are properly published to the run, so we shouldn't have to do this ourselves; however, the display is rather minimal (shown only as a couple lines in Checks) so we may want to consider reopening this to improve with a comment if devs are having difficulty finding coverage reports.

/cc @AlexGhiondea