Updater is broken for 10.0a5 with Error: 22

We have reports that the alpha update is broken for some folks, resulting in something like

PATCH DIRECTORY /home/tb/tor-browser/Browser/TorBrowser/UpdateInfo/updates/0
INSTALLATION DIRECTORY /home/tb/tor-browser/Browser
WORKING DIRECTORY /home/tb/tor-browser/Browser
failed: 22

which points to a MAR channel mismatch. dgoulet had the same problem and indeed it turned out that his update-settings.ini showed

ACCEPTED_MAR_CHANNEL_IDS=torbrowser-torproject-release

He had been updating the alpha for years but was still on torbrowser-torproject-release. After manually setting that to the alpha channel the update works.

I wonder why the IDS never changed.

@mcs, @brade