Fix when the env variable was None

Author: martinRenou

jupyterlite_xeus_python/env_build_addon.py

@@ -247,6 +247,9 @@ def post_build(self, manager):
         shutil.rmtree(self.cwd.name, ignore_errors=True)
         shutil.rmtree(self.root_prefix, ignore_errors=True)
 
-        os.environ["CONDARC"] = self.orig_config
+        if self.orig_config is not None:
+            os.environ["CONDARC"] = self.orig_config
+        else:
+            del os.environ["CONDARC"]
 
         return []