GH Actions: fix use of deprecated `set-output`

[jrfnl]

GitHub has deprecated the use of set-output (and set-state) in favour of new environment files.

This commit updates workflows to use the new methodology.

Refs:

• https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
• https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#environment-files

[jrfnl]

As the build is now passing, I will merge this as otherwise we'll have failing builds again once GH completely stops supporting set-output.