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

@@ -23,6 +23,12 @@ variables:
   ## Ref: https://github.com/eic/eic-spack/commit/[hash]
   ## [hash]: [description]
 
+  ## S3 buildcache access
+  S3_ENDPOINT_URL: "https://dtn01.sdcc.bnl.gov:9000"
+  ## S3 credentials (through CI/CD variables)
+  AWS_ACCESS_KEY_ID: "${S3_ACCESS_KEY}"
+  AWS_SECRET_ACCESS_KEY: "${S3_SECRET_KEY}"
+
   ## We need to enable Docker Buildkit to use cache mounts and better
   ## build performance overall
   DOCKER_BUILDKIT: 1