This patch adds the unsafe-cache option to run.py and changes mkzfs.py
to always call run.py with this option enabled.
Thus, we're doing this change just for the build run (Suggested by Nadav Har'El).
The main goal is to boost the time it takes to complete the entire process.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>