fix: Update deprecated `set-output` format for GitHub Actions to use environment variables

[sevenseacat]

See https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/ for more details

Closes #60

(Note: I have merely proved it correct, not tried it)

Contributor checklist

• [x] My commit messages follow the Conventional Commit Message Format For example: fix: Multiply by appropriate coefficient, or feat(Calculator): Correctly preserve history Any explanation or long form information in your commit message should be in a separate paragraph, separated by a blank line from the primary message
• [ ] Bug fixes include regression tests
• [ ] Features include unit/acceptance tests

[sevenseacat]

Actually... I think I need to try it because I'm not sure it will work right now

[sevenseacat]

So I think this is right now. According to the blog, we can write to the file in GITHUB_OUTPUT, with the format {key}={value} so that's what I've done. It works in my testing!