ci(runners): specify GitHub Actions Ubuntu runner image

[d33bs]

Description

This PR addresses #410 by adding a specific Ubuntu runner image to help avoid issues we saw from #406 and #408 and keep things declarative within the project. I also tried to find a way to set a default or use a variable, but it seems this is a bit of a challenge generally (so as to set in one spot instead of many). Let me know if you know of a better route towards this!

Thanks for any thoughts or feedback you may have!

Closes #410

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• [ ] Bug fix (fixes an issue).
• [x] Enhancement (adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
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[d33bs]

Thanks @kenibrewer ! Merging this in.