Windows: Update default config for stage location (#48511)

Current location is within the Spack prefix, which causes builds to
pollute VCS with stage artifacts and generally inflates the Spack
install prefix.

This PR moves it to the user cache location now that we can
consistently support paths with spaces on Windows.

etc/spack/defaults/windows/config.yaml

@@ -1,5 +1,5 @@
 config:
   locks: false
   build_stage::
-    - '$spack/.staging'
+    - '$user_cache_path/stage'
   stage_name: '{name}-{version}-{hash:7}'