Add a CI-job to run quick-check tests in OPT mode, to ensure Docker image builds don't regress. #292

This issue is a follow-on to issue issue #287 to ensure that Docker images are correctly built, so that we don't run into the following inconsistent behaviour. (Following extracted from that other issue's description.)

Add a CI-job to run quick-check scripts in optimized mode, for future coverage against regression in CI-scripts.

Looking at the latest CI results from your PR, I notice that the 3 jobs which are failing are the non-debug jobs, namely ci/clang, ci/gcc and ci/rust.
Looking at the top-level job definitions here I see that those are the only three jobs which call the steps_pr_build_test function in the ytt template. Other jobs call other functions like steps_pr_debug_build_test
Comparing the steps_pr_build_test versus steps_pr_debug_build_test in the ytt functions file, I see that the steps_pr_build_test job first builds a docker image, and then runs tests inside the built image. By contrast, the steps_pr_debug_build_test job does a build-and-test in a single step, within the build-env image.