Migrate to GitHub Actions

[kmoe]

CircleCI has some limitations:

• Slack build notification functionality is verbose and inflexible
• config file is verbose in general (see removals in CI: Build and test last two stable Go versions (1.16 and 1.17) #216)

We only added build failure Slack notifications to the final job in the nightly workflow, leading to the nightly failure #219 going unnoticed for a few days.

Other HashiCorp projects are converging on GitHub Actions as the preferred CI solution. Explore this possibility for terraform-exec and migrate if it has everything we need.

[kmoe]

This may be worth doing in conjunction with #318: use a script in the test job to cache the Terraform binaries beforehand.