comms: fix SSH comms

* SSH->TCP client was missing the `ssh forward environment variables`
  key.
* This PR gives it an empty placeholder value for now.

cylc/flow/network/ssh_client.py

@@ -67,6 +67,7 @@ async def async_request(
                     'ssh command': ssh_cmd,
                     'cylc path': cylc_path,
                     'use login shell': login_sh,
+                    'ssh forward environment variables': [],
                 }
                 # NOTE: this can not raise NoHostsError
                 # because we have provided the host