_byarch channels appear to be broken

[Dimitri John Ledkov]

In [21]: snap=lp.load('~canonical-kernel-snaps/+snap/noble--linux--pc-kernel--edge--1+2')

In [22]: snap.auto_build_channels = {'_byarch': {'riscv64': {'snapd': 'edge'}}}

In [23]: snap.lp_save()

BadRequest: HTTP Error 400: Bad Request
Response headers:
---
connection: close
content-length: 50
content-security-policy: frame-ancestors 'self';
content-type: text/plain;charset=utf-8
date: Fri, 16 Feb 2024 22:28:34 GMT
server: gunicorn
status: 400
strict-transport-security: max-age=15552000
vary: Accept
x-content-type-options: nosniff
x-frame-options: SAMEORIGIN
x-launchpad-revision: 9643586c585856148a18782148972ae9c1179d06
x-lazr-notifications: []
x-powered-by: Zope (www.zope.org), Python (www.python.org)
x-request-id: ebf6a6b8-4fc5-4542-b561-c6dba3e71231
x-vcs-revision: 9643586c585856148a18782148972ae9c1179d06
x-xss-protection: 1; mode=block
---
Response body:
---
b"auto_build_channels: '_byarch' isn't a valid token"
---

but this used to work as per https://chat.canonical.com/canonical/pl/zqqaaiu3i3fa9y76h314dosxyc