physical root is not mounted after bootc upgrade on s390x

[shi2wei3]

We test s390x for downstream images due to the testing-farm support, and we found s390x system failed to boot after bootc upgrade, it happens to developement and GA releases.

I reproduced on centos-bootc:stream10, and find after reboot, the /sysroot folder is empty when ostree-prepare-root try to access files in this folder, and it's not mounted.