downstream consumer tests are no longer run?

[tmadlener]

I just realized this in #230. It looks like the https://github.com/key4hep/k4FWCore/tree/main/test/downstream-project-cmake-test is no longer built in CI (and it has been like this for quite some time?)

I think in this project it would make sense to ensure that we don't break the downstream cmake config accidentally in CI.

edit: we technically have the downstream workflow that tests all currently used real world features, but given it's frequent unrelated failures, I think it would be useful to have a dedicated test

edit: We have the downstream build workflow that should cover all "real world use cases", but given that that has been rather flaky recently, I still think a dedicated small workflow (or addition to an existing workflow) would make sense.