Do not create PDF on forks

[nuest]

Let's not run the action on forks, so we do not create each PDF twice.

For testing, one can run the code form the action manually.

[nuest]

@Fmazin You're the actions expert :-) This should not take long, right?

[Fmazin]

The easiest way i found was to to check the repo owner and if it is not o2r-project the action will not start. Is that an okay solution?