Actions ran from forks cannot access repository secrets

[rjaegers]

When forking this repository and opening a pull request from the fork, it is possible to run the GitHub Actions after approval. This run however cannot acces repository secrets. In practice this means that at least the SonarCloud and Fuzzer jobs are not able to complete successfully from a fork.

This needs a solution that still guards the quality of all commits that end up on one of the main branches.