Use GITHUB_ENV instead of set-env

[peaceiris]

https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Action and workflow authors who are setting environment variables via stdout should update any usage of the set-env and add-path workflow commands to use the new environment files.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

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[MikeMcQuaid]

Thanks again @peaceiris!