Flathub buildbot build stopped working


I am the maintainer of the Betterbird flatpak. The last flatpak I shipped was built successfully on 27 May. I have not changed the manifest since then, but when I retry the build today (PR), it fails (Build log).

Has there been any change to the build environment recently? Do you have any suggestions how to debug this?

EDIT: It seems that org.freedesktop.Sdk.Extension.rust-stable has been updated to provide rust 1.70.0 since 1 June; before it provided rust 1.69.0. Maybe this unexpected update causes the issues. Is there any way to use a fixed version of rust for building the flatpak instead of a floating “stable” version?