Switch back to using pivot_root

[deliciouslytyped]

xred #23 , xref probably #102
@jD91mZM2

pivot_root should probably be used over chroot. I don't understand why pivot_root was used at some point originally, and then switched back.

The unshare man page says:

       EPERM (since Linux 3.9)
              CLONE_NEWUSER was specified in flags and the caller is in
              a chroot environment (i.e., the caller's root directory
              does not match the root directory of the mount namespace
              in which it resides).

This prevents using unshare in nix-user-chroot, which would be nice to have working.

Apparently container solutions also normally use pivot_root instead of chroot.