Use site_url for the rewrite_rule when alternate_redirect_uri is true

[StaymanHou]

All Submissions:

• [x] Have you followed the plugin Contributing guideline?
• [x] Does your code follow the WordPress' coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

For better consistency, use site_url for the rewrite_rule instead of hard-coding the path when alternate_redirect_uri is true.

The change is made primarily to fix a problem when WP_SITEURL is set to a subdirectory while alternate_redirect_uri is set to true.

How to test the changes in this Pull Request:

1. Set WP_SITEURL to a subdirectory
2. Check the Alternate Redirect URI option in settings
3. Attempt a login with OpenID Connect