Match By Email should be disabled

Since we hardcode the set by email option, the checkbox should be disabled. After checking in wp-cli, `Jetpack_SSO_Helpers::is_match_by_email_checkbox_disabled()` returns the correct value. It seems like `render_match_by_email()` is not firing, but I'm not sure where that field is coming from then.

https://github.com/Automattic/jetpack/blob/5c4fd63e63cdcb380a16d33a591a265ca6b4318e/modules/sso.php#L291

Start in modules/sso.php around the linked line and trace how the Match By Email field is rendered, comparing that path with Jetpack_SSO_Helpers::is_match_by_email_checkbox_disabled() and render_match_by_email(). Done means the checkbox is disabled when the email option is hardcoded and the existing wp-cli check remains correct.

Written by the indexing model from the issue text.