bubblewrap should fall back to MS_MOVE if pivot_root() fails

`pivot_root()` does not work in the initramfs. As a fallback, `MS_MOVE` can be used together with `chroot()` to mimick `pivot_root()` somewhat. bubblewrap should use this fallback if `pivot_root()` is not possible (e.g. when running bubblewrap from the initramfs) so that bubblewrap can be used from the initramfs.

Currently running bubblewrap from the initramfs fails with `bwrap: pivot_root: Invalid argument`