reproduce (this is a W=1 build):
        git remote add bcachefs https://evilpiepirate.org/git/bcachefs.git
        git fetch --no-tags bcachefs master
        git checkout e4d5b96272266d46c64205669dfcb03315d1859a
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash