CI: Move the build stage to the project root instead of tmp (#47996)

On MacOS the tmpdir fills up over time and isn't cleared properly.
Move the build stage to a location it is cleared after each build.

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

@@ -5,4 +5,6 @@ config:
     padded_length: 256
     projections:
       all: '{architecture}/{compiler.name}-{compiler.version}/{name}-{version}-{hash}'
+  build_stage:
+  - $spack/tmp/stage