ci: switch to GitHub Actions

[kyleam]

This PR switches the CI from Drone to GitHub Actions. Summary of keys differences:

• OS
  • new: Ubuntu 20.04 and 22.04
  • old: custom image based on Ubuntu 18.04
• R
  • new: 3.6, 4.0, 4.1, and latest R (currently 4.4.0)
  • old: 3.6, 4.0, and 4.1

---

• [x] Drop tip commit that tests dry-run s3 sync.

[kyleam]

Test s3 auth and dry-run upload look good:

• https://github.com/metrumresearchgroup/yspec/actions/runs/8943085443/job/24567186431
• b1ed13feaae2a776d051ea40f5ec6b360efd710a

I'll drop the tip commit.