Symfony version(s) affected: 4.4.x-dev
Description
Since #34069 the sync:// transport is handled differently.
The case when the transport name is defined using an environment variable is broken.

Works:

framework:
   messenger:
     transports:
       default: "sync://"

Broken case:

parameters:
  env(MESSENGER_TRANSPORT): sync://

framework:
   messenger:
     transports:
       default: "%env(MESSENGER_TRANSPORT)%"

Yields No transport supports the given Messenger DSN "sync://" when dispatching a message.

How to reproduce
See https://github.com/Devristo/sf4.4-messenger-bug for a reproducer.

Possible Solution

• Document this BC-break
• Fix the BC-break: If someone could give me some pointers I am willing to implement a fix.