ci: Increase the amount of pruning possible for PR pipelines (#35944)

By setting the traversal depth to 1, only specs matching the changed
package and direct dependents of those (and of course all dependencies
of that set) are removed from pruning candidacy.

share/spack/gitlab/cloud_pipelines/.gitlab-ci.yml

@@ -27,6 +27,7 @@ default:
     SPACK_BUILDCACHE_DESTINATION: "s3://spack-binaries-prs/${CI_COMMIT_REF_NAME}/${SPACK_CI_STACK_NAME}"
     SPACK_PIPELINE_TYPE: "spack_pull_request"
     SPACK_PRUNE_UNTOUCHED: "True"
+    SPACK_UNTOUCHED_PRUNING_DEPENDENT_DEPTH: 1
 
 .protected-refs:
   only: