When the registration_shared_secret changes (derived from
matrix_synapse_macaroon_secret_key), a running Synapse container still
has the old secret in its config. This causes register_new_matrix_user
to fail with "HMAC incorrect" when the matrix-user-creator role tries
to register users.

This mirrors the approach from 2a581cce (which added similar retry
logic for the Matrix Authentication Service on database auth failure):
if the initial registration attempt fails with an HMAC error, restart
Synapse (picking up the new config with the updated secret), wait for
it to start, and retry.

Caused by c21a80d232

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>