freedesktop-sdk: Backport fusermount symlink

Flatpak is broken since we dropped fuse2 dependency because it calls fusermount instead of fusermount3. The symlink will be added in freedesktop-sdk fix in https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/merge_requests/13490 Until we can switch freedesktop-sdk to master, this is a backport of that fix.