Backport v2025 <- v2026dev1: Disable use of flathub for armhf

Since flathub does not support armhf anymore, avoid using it to make our pipelines green. A proper solution will be to build and publish our own version of rhythmbox3.

Signed-off-by: Walter Lozano walter.lozano@collabora.com

Backport of !699 (merged)