GitHub Actions: new way to print to stdout

Currently our Workflow throws this 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/
```
We have to adjust the way we write to stdout in order to fix that warning.