Redesign level venture capital to prevent attacks from other players

[StillFantastic]

trafficstars

If some players send ethers to addresses of future instances could lead to other players unable to win.

https://github.com/cypherpunks-core/cypherpunks-ctf/blob/aa4c2500aaddfa8941780546bb5afb177c5c2b9c/contracts/levels/VentureCapital.sol#L14-L15