Add scope to setting active tree to ensure that it is set at user level

lib/spack/spack/package.py

@@ -1499,12 +1499,14 @@ def do_install(self, **kwargs):
             global_root = spack.config.get('upstreams')
             global_root = global_root['global']['install_tree']
             global_root = spack.util.path.canonicalize_path(global_root)
-            spack.config.set('config:active_tree', global_root)
+            spack.config.set('config:active_tree', global_root,
+                             scope='user')
         elif upstream:
             raise NotImplementedError
         else:
             spack.config.set('config:active_tree',
-                             spack.config.get('config:install_tree'))
+                             spack.config.get('config:install_tree'),
+                             scope='user')
 
         # For external packages the workflow is simplified, and basically
         # consists in module file generation and registration in the DB