fix(gha): Avoid running cron on forks

[Altair-Bueno]

The cron job of Ditto is triggered each day on forks, which leads to annoying failed jobs (see https://github.com/ertis-research/ditto/actions/runs/5459864259/jobs/9936273159, for example). Solution is based on the following sources:

• https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idif
• https://github.com/orgs/community/discussions/26684#discussioncomment-3252843

PS: This change also triggered maven tests, which failed. See https://github.com/ertis-research/ditto/actions/runs/5461992872/jobs/9940730952. It may be wise to add .github to the list of ignored paths on maven.yml, and identify the reason why it failed.

[thjaeckle]

Thanks @Altair-Bueno
Very valid PR .. did not think about that.

You could probably also deactivate GH Actions for your fork completely - but I agree that the PR makes sense.