Cirrus CI: Trigger on pull requests or downstream repos

Since Cirrus Labs is limiting their free usage tier [1], limit CI runs
on pull requests only. Otherwise, we might deplete our monthly quota
within a few days.

Adapt the task amd64-llvm16 to execute on downstream repos or on pull
requests only.

Other alternatives will be further studied.

[1]: https://cirrus-ci.org/blog/2023/07/17/limiting-free-usage-of-cirrus-ci/