Add step to get latest hugo theme version and set as ENV var

[nginx-jack]

Proposed changes

Gets the latest hugo theme version at workflow run time. This means we will not have to update all action callers when we have a new hugo theme.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[mjang]

Question, @nginx-jack . I was looking at this line, and wondering about the purpose of $GITHUB_ENV. (Why do we add the latest Hugo version number, among other things, to that variable.)

[lamATnginx]

LGTM