Github Actions: `set_output` is deprecated, update to new standard

[PGijsbers]

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In the 'Store repository status' step we get this message:

Run echo "::set-output name=BEFORE::$(git status --porcelain -b)" Warning: The set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

[PGijsbers]

Not necessary to do this for a release since it doesn't affect the distribution, but lumping it in the milestone for easier reference in the hackathon.