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

CMakeLists.txt

@@ -1265,6 +1265,12 @@ if (LLAMA_METAL)
             GROUP_READ
             WORLD_READ
         DESTINATION ${CMAKE_INSTALL_BINDIR})
+    if (NOT LLAMA_METAL_EMBED_LIBRARY)
+        install(
+            FILES ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/default.metallib
+            DESTINATION ${CMAKE_INSTALL_BINDIR}
+        )
+    endif()
 endif()
 
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