Remove set-env in favor of environment variables file

[B0go]

As of October 1, Github deprecated the set-env command in favor of environment variables files: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

All actions using set-env from now on produce a warning while executing as can be seen in this image:

This PR updates the current example and test workflow to use environment files

Environment files docs: https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#setting-an-environment-variable

Signed-off-by: B0go victorbogo@icloud.com

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