Allow users to specify root env dir

Environments managed by spack have some advantages over anonymous Environments
but they are tucked away inside spack's directory tree. This PR gives
users the ability to specify where the environments should live.

See #32823

etc/spack/defaults/config.yaml

@@ -76,6 +76,10 @@ config:
   source_cache: $spack/var/spack/cache
 
 
+  ## Directory where spack managed environments are created and stored
+  environments_root: $spack/var/spack/environments
+
+
   # Cache directory for miscellaneous files, like the package index.
   # This can be purged with `spack clean --misc-cache`
   misc_cache: $user_cache_path/cache