Fix bundling AppImageUpdate into appimages

Commit de2315ef added the check to disable embedding the AppImageUpdate
script when CHANNEL is not manually set from the command line. However,
this breaks all the Plus and Release builds on the Binary Factory (which
use CHANNEL autodetection from the source code).

AppImageUpdate is available only for amd64 at this moment.

BUG:446757
(cherry picked from commit 7e1dca68)