make-bootstrap-tools.nix test: Use busybox from store

Our bootstrap tools are actually broken right now due to busybox not
working when invoked directly from a store path. (It says e.g.
"0qqqw19y4gmknajw8vg4fvhx9gxdqlhz-busybox: applet not found").
Make this test actually fail in such case, the next commit will fix the
problem with busybox.