vagrant: Cache cfssl binaries
vagrant: Cache cfssl binaries

Those binaries are a few MB large and caching them cuts a few seconds
from the total provisioning time.

Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

tgraf/cilium,cilium/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,cilium-team/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium