iOS Simulator Metal shader is built with the wrong SDK

[zhaofengli]

On the iOS Simulator, the terminal fails to initialize with:

Compiler failed to build request
metal error=Target OS is incompatible: library was not compiled for the simulator
error initializing surface err=error.MetalFailed

I have prepared #7636 (https://github.com/zhaofengli/ghostty/tree/fix-simulator-metallib) to fix this.

[mitchellh]

Fixed via the PR. Thanks.