Use values `Environment secrets` in code

### This issue affects

- [x] The site generated by GitHub Pages
- [ ] Building sites locally

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

### What did you do (e.g., steps to reproduce)

I have read the following information.
* Link to the official news: https://github.blog/changelog/2020-12-15-github-actions-environments-environment-protection-rules-and-environment-secrets-beta/
* Link to the GitHub Docs: https://docs.github.com/en/actions/reference/encrypted-secrets

### What did you expect to happen?

* Using `.evn` variables from repository settings

### What happened instead?

* There is no possibility of using `.evn` variables in the code