Fix contrib/vagrant/Makefile to work on non bash-symlinked /bin/sh.

The current `contrib/vagrant/Makefile` won't `make build` when `/bin/sh` is not symlinked to `/bin/bash` (as on standard Ubuntu), because the `[[ .. ]]` and `(( .. ))` syntax are bash-specific. This PR rewrites the loop using standard `/bin/sh` syntax.