Launch: Fix logic to compute redirectToWpcomPath origin

[ciampo]

Changes proposed in this Pull Request

• Fix the logic for computing the origin in the redirectToWpcomPath function

Testing instructions

Setup

• Create a site with /start and add it to the sandbox (do not launch it)
• Enable Focused Launch flow A/B testing experiment

Test in Calypso

• Click "Launch" in the block editor on Calypso
• Click on "View all domains"
• Click on "Use a domain I own"
  • You should be redirected correctly to the domain transfer launch flow
• Click on the back link on the top left of the page
  • You should be redirected to the block editor
• Open Focused Launch modal again, and pick free subdomain and a paid plan
• Click Launch
  • You should be redirected to the correct checkout page

Test in WP Admin

Repeat steps above, but from the WP Admin block editor instead

Related to #50242

[matticbot]

Test live: https://calypso.live/?branch=fix/launch-redirect-to-wpcom-base-url

[StefanNieuwenhuis]

Changes LGTM! 🚢

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[matticbot]

Caution: This PR affects files in the Editing Toolkit Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D57284-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing the Editing Toolkit Plugin for more info: PCYsg-ly5-p2