Fix "Create Form" link destinations when editing edit forms

Summary:
Fixes T12301. In D17372, this changed to use generic EditEngines instead of the proper runtime engine. Normally this doesn't matter, but can in this case.

After loading the configurations normally, swap their attached engines for the specific configured runtime engine we're currently executing.

Test Plan: Clicked "Create Form" from the Maniphest form list, saw it go to "Create Maniphest Form", not "Create Generic Meta-Form".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12301

Differential Revision: https://secure.phabricator.com/D17398