set-output is deprecated

[georg-jung]

When this action runs, one gets a warning:

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/

[rix1337]

@lucacome I just proposed a change that has worked in my other repos to alleviate the deprecation notice.
Needs your testing, but should fix this issue.