Use site_url for the rewrite_rule when alternate_redirect_uri is true

[StaymanHou]

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• Does your code follow the WordPress' coding standards?
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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