.gitlab-ci.yaml: f-branches try to build upon incorrect images

Summary

Currently, f-branches try to build upon incorrect images. For example, f-fix-async-timeout tries to use the Docker image gitlab.indiscale.com:5050/caosdb/src/caosdb-cppinttest/testenv:f-fix-async-timeout, which does not exist.

See this job.

Possible fixes

Look at .gitlab-ci.yaml in the pyinttests. It looks as if $CPPINTTEST_REGISTRY_IMAGE and $CPPINTTEST_REGISTRY_IMAGE were confused.