Keep the 5.1 branch buildable

[Halla Rempt]

Hi,

Because krita/5.1 is still our designated stable branch, we need to keep it green both on binary factory and in CI. I'm not sure what caused the failure of https://binary-factory.kde.org/job/Krita_Stable_Appimage_Build/ -- maybe it was just a glitch, so I've restarted that, but there's also https://invent.kde.org/graphics/krita/-/jobs/695658#L6025.

Halla