Install cpu-optimized version of Stockfish

[fitztrev]

Determine which CPU architecture client is using and install the optimized version from the engine directory.

Right now, it's just installing the default version for each OS.

This is the line that needs to be changed.

[niklasf]

Here's how fishnet does this: https://github.com/lichess-org/fishnet/blob/d0990ef62422c86013270e768e5cb8724f4585f7/src/assets.rs#L98-L131. It's missing some of the weaker targets, which are not expected to run fishnet, but that might run this client.