.gitlab-ci.yml: Always build CI test/lint images

This commit removes the need to use Trusted Runners for building CI
test/lint images. Instead, the default kubernetes) runners are used.
The kubernetes runners have access to a registry that is specifically
intended to be used temporarily during CI pipelines. The
build-conda-env-test-image and build-image-conda-env-lint jobs now
always run to prepare suitable images for subsequent jobs in the
pipeline, and they publish to the aforementioned registry. The
registry-based buildkit caching available with the kubernetes runners
allows this build process to avoid redoing work unless specific files
(those mentioned as requirements in docker/blubber.yml) are changed.