Updated GitHub Actions Environments

[karbassi]

Rather than *-latest, I'm specifying specific environments.

List of environments: https://github.com/actions/virtual-environments#available-environments

Related to #369

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Before submitting a pull request, please make sure the following is done:

• [x] Fork the repository and create your branch from master.
• [ ] If you've added code that should be tested, add tests!
• [ ] Ensure the test suite passes.
• [x] Format your code with ShellCheck.
• [x] Include a human-readable description of what the pull request is trying to accomplish.
• [x] Steps for the reviewer(s) on how they can manually QA the changes.
• [x] Have a fixes #XX reference to the issue that this pull request fixes.

[karbassi]

Another question to figure out is why make takes 10-14 seconds on macOS, while it takes basically nothing on Ubuntu.