That’s weird. While there was an issue with 3.6.6, I was able to update from 3.6.5.1 to 3.6.6.1 in another site.
$ wp plugin update code-snippets
Enabling Maintenance mode...
Downloading update from https://downloads.wp.xz.cn/plugin/code-snippets.3.6.6.1.zip...
Unpacking the update...
Installing the latest version...
Removing the old version of the plugin...
Plugin updated successfully.
Disabling Maintenance mode...
+---------------+-------------+-------------+---------+
| name | old_version | new_version | status |
+---------------+-------------+-------------+---------+
| code-snippets | 3.6.5.1 | 3.6.6.1 | Updated |
+---------------+-------------+-------------+---------+
Success: Updated 1 of 1 plugins.
