ci: switch to GitHub Actions

[kyleam]

This series switches the CI from Drone to GitHub Actions.

• The main switch happens in the third commit.

• The first commit tightens the install_torsten() skips to hopefully avoid gh-125. This isn't directly related, but I'm trying to avoid spurious failures as iterating/troubleshooting this switch to GitHub Actions.

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

[kyleam]

Test s3 auth and dry-run upload look good:

• https://github.com/metrumresearchgroup/bbr.bayes/actions/runs/8901716835/job/24446525873?pr=142
• b488248878b70cb346b85b1cf41a96c831cdc271

I'll drop the tip commit.

[kyleam]

Pushed update to pull shared actions from new repo at https://github.com/metrumresearchgroup/actions.