Reset active directory after each global install

lib/spack/spack/cmd/install.py

@@ -200,6 +200,8 @@ def install(spec, kwargs):
             env.write()
         else:
             spec.package.do_install(**kwargs)
+            spack.config.set('config:active_tree', '~/.spack/opt/spack',
+                             scope='user')
 
     try:
         if cli_args.things_to_install == 'dependencies':