Enable GitHub Actions

[ngie-eign]

In order for the change in https://github.com/freebsd/atf/pull/93 to be made effective for all contributing PRs, GitHub Actions will need to be turned on for the repo.

I recommend the following settings:

• [ ] Allow all actions and reusable workflows (radio button selected).
• [ ] Require approval for first-time contributors (radio button selected).
• [ ] Read repository contents and packages permissions (radio button selected).
• [ ] Allow GitHub Actions to create and approve pull requests (checkbox unchecked).

If/when releases become automated, it makes sense to check "Allow GitHub Actions to create and approve pull requests", but right now it doesn't make sense and we should use principle of least privilege.