feat: use spack ci generate to setup build pipeline

Rather than having a single long build, we can split it up in different jobs with spack ci generate. This will be more resilient against failures. We can also run the builds inside a regular job instead of inside the container, and then only pull from the build cache once we build the container.

.gitlab-ci.yml

@@ -217,8 +217,11 @@ generate-pipeline:
   stage: generate
   tags:
     - docker-new
+  parallel:
+    matrix:
+      - BASE_IMAGE: debian_testing_base      
   image:
-    name: eicweb.phy.anl.gov:4567/containers/eic_container/debian_testing_base
+    name: ${CI_REGISTRY_IMAGE}/${BASE_IMAGE}:${INTERNAL_TAG}
     entrypoint: [""]
   before_script:
     - git clone ${SPACK_REPO} spack