Run the FOSS-specs without the ee code in gitlab-org/gitlab

When I merged !16117 (comment 221903576), it contained a reference to an EE-class in a FOSS-file.

This did not cause any issues in the EE-pipelines, but caused a master:broken in gitlab-org/gitlab-foss: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/303730150

Should and could we run a set of pipelines of only the FOSS code within the merge request to gitlab-org/gitlab if there are non-ee files that are changed? That way we would have discovered this sooner, at the cost of some compute.