* Symlink to /usr/bin/xcrun so that `xcrun` binary
is usable during build (used for compiling Metal shaders)
Fixes https://github.com/ggerganov/llama.cpp/issues/6117
* cmake - copy default.metallib to install directory
When metal files are compiled to default.metallib, Cmake needs to add this to the install directory so that it's visible to llama-cpp
Also, update package.nix to use absolute path for default.metallib (it's not finding the bundle)
* add `precompileMetalShaders` flag (defaults to false) to disable precompilation of metal shader
Precompilation requires Xcode to be installed and requires disable sandbox on nix-darwin
e190f1fca6