ci: remove --keep-stage flag (#49467)

logs are now copied from the install dir

share/spack/gitlab/cloud_pipelines/configs/darwin/aarch64/config.yaml

@@ -1,5 +1,3 @@
 config:
-  build_stage:
-  - $spack/tmp/stage
   install_tree:
     root: $spack/opt/spack

share/spack/gitlab/cloud_pipelines/configs/darwin/ci.yaml

@@ -8,13 +8,5 @@ ci:
       tags: [spack]
   - build-job:
       tags: [ "macos-sequoia", "apple-clang-16", "aarch64-macos" ]
-
-      # after_script intended to ensure all stage files are properly cleaned up,
-      # including those that may have been created as read-only by `go mod`
-      # as part of installation of a golang package
-      # see: https://github.com/spack/spack/issues/49147
-      after_script-:
-      - - if [[ -d tmp ]] ; then chmod -R u+w tmp ; else echo tmp not found ; fi
-        - ./bin/spack clean -a
   - build-job-remove:
       image:: macos-run-on-metal