Restore matrix_mautrix_signal_appservice_bot_username usage

Fixes #3088

Looks like the migration to the Go-based Signal bridge hardcoded the
`signalbot` username instead of using the variable we had.
Related to: #3041

roles/custom/matrix-bridge-mautrix-signal/templates/config.yaml.j2

@@ -58,7 +58,7 @@ appservice:
     # Appservice bot details.
     bot:
         # Username of the appservice bot.
-        username: signalbot
+        username: {{ matrix_mautrix_signal_appservice_bot_username | to_json }}
         # Display name and avatar for bot. Set to "remove" to remove display name/avatar, leave empty
         # to leave display name/avatar as-is.
         displayname: signal bridge bot