Expand external find for Windows (#27588)

* Incorporate new search location

* Add external user option

* proper doc string

* Explicit commands in getting started

* raise during chgrp on Win

recover installer changes

Notate admin privleges

Windows phase install hooks

Find external python and install ninja (#23496)

Allow external find python to find windows python and spack install ninja

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>
Co-authored-by: Betsy McPhail <betsy.mcphail@kitware.com>

etc/spack/defaults/windows/config.yaml

@@ -2,4 +2,4 @@ config:
   locks: false
   concretizer: original
   build_stage::
-    - '~/.spack'
+    - '$spack/.staging'