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

@@ -236,7 +236,7 @@ generate-pipeline:
     - git clone https://github.com/${EICSPACK_ORGREPO}.git ${SPACK_ROOT}/var/spack/repos/eic-spack
     - git -C ${SPACK_ROOT}/var/spack/repos/eic-spack checkout ${EICSPACK_VERSION}
   script:
-    - spack env activate --without-view .
+    - spack -d env activate --without-view .
     - spack -d ci generate
       --artifacts-root "${CI_PROJECT_DIR}/jobs_scratch_dir"
       --output-file "${CI_PROJECT_DIR}/jobs_scratch_dir/pipeline.yml"