Setup.sh: Find a better way to install g++-7 as the current approach fails some times due server keys and permisions

[xavisolesoft]

[xavisolesoft]

Seems there is no automated solution for the focal key issue.

[xavisolesoft]

I tried to use the g++ toolchain from inside unreal but it's too old (g++ 4) and anyway is not working.

[xavisolesoft]

We should consider the option about having our own g++ embeded inside carla to prevent conflicts with contributors local environments.