3C build status badge points to deleted workflow

[mattmccutchen-cci]

The 3C build status badge in the root README.md points to the workflow in the checkedc-clang repository, which is now deleted. If we want to use the badge from the new actions repository, we'll have to make that repository public. That should be safe, right? The repository contains nothing confidential, and the action runs only on the default branch of checkedc-clang and only on schedule or when triggered by someone with write access to the actions repository, so there's no way for someone to run malicious code by submitting a pull request.

[mattmccutchen-cci]

This would be nice to have for our upcoming PR to Microsoft if it is as easy as I think it is. @mwhicks1 Is it OK to make the actions repository public? (If we want to run private actions in the future, we can just make a separate actions-private repository.)

I realize it may be a bit confusing if the badge appears in Microsoft's repository but reflects the (usually different) version of 3C in our repository. I don't think we ever had a real solution to this: we added the badge to our checkedc-clang repository back when it contained the action, but we moved the action before the addition of the badge was ever submitted to Microsoft. Is this concern serious enough to merit removing the badge?

As an aside, Microsoft's own build status badges currently seem to be broken, but they can fix that at their leisure.

[mattmccutchen-cci]

We decided it is OK to make the repository public, and Anwar did so. I'll update the badge.