gitlab-ci: enable assertions on fedora34-ninja jobs

We should have at least one CI job in merge request pipelines that
builds CMake with assertions enabled.  We avoid using the `Debug`
configuration in order to keep CI artifacts small, so instead use
the `Release` configuration without `-DNDEBUG`.
35 jobs for !6927 with ci-assertions in 32 minutes and 38 seconds (queued for 3 seconds)
latest merge request