ci: Update for GitHub Actions v2.274.2

set-env and add-path have been removed since GitHub Actions v2.274.2. [1]
Switch to Environment Files instead. [2]

[1] https://github.blog/changelog/2020-11-09-github-actions-removing-set-env-and-add-path-commands-on-november-16/
[2] https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#environment-files