Fix personality switching from x86_64 to i686

On Linux, Nix can build i686 packages even on x86_64 systems.  It's not
enough to recognize this situation by settings.thisSystem, we also have
to consult uname().  E.g. we can be running on a i686 Debian with an
amd64 kernel.  In that situation settings.thisSystem is i686-linux, but
we still need to change personality to i686 to make builds consistent.