Maybe it is too late to write about this, but nothing is too late.
In early days, GitHub Actions supports HCL format, but it was deprecated. GitHub deprecated it on Sep 30. Here is the announcement from https://developer.github.com/actions/
The documentation at https://developer.github.com/actions and support for the HCL syntax in GitHub Actions will be deprecated on September 30, 2019. Documentation for the new limited public beta using the YAML syntax is available on https://help.github.com. See “Automating your workflow with GitHub Actions” for documentation using the YAML syntax.
Now the only supported syntax is in YAML format. To learn more about it, you can read this page: https://help.github.com/en/actions/automating-your-workflow-with-github-actions
tags: github-actions