nix: make `xcrun` visible in Nix sandbox for precompiling Metal shaders (#6118)

* 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