gitlab ci: Make sure PR pipelines can push to buildcache (#32014)

On PR pipelines we need to override the buildcache destination to
point to the "spack-binaries-prs" bucket, otherwise, those pipelines
try to push to the default mirror in a bucket for which they don't
have write permission.

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

@@ -193,6 +193,7 @@ e4s-mac-pr-generate:
     - cd share/spack/gitlab/cloud_pipelines/stacks/${SPACK_CI_STACK_NAME}
     - spack env activate --without-view .
     - spack ci generate --check-index-only
+      --buildcache-destination "${SPACK_BUILDCACHE_DESTINATION}"
       --artifacts-root "${CI_PROJECT_DIR}/jobs_scratch_dir"
       --output-file "${CI_PROJECT_DIR}/jobs_scratch_dir/cloud-ci-pipeline.yml"
   artifacts: