Fix CI: let MLX fetch fmt via FetchContent

USE_SYSTEM_FMT=ON required a system-installed fmt library,
which the CI runner doesn't have. Remove the flag so MLX's
cmake fetches fmt itself.

Author: ausimian

mix.exs

@@ -197,7 +197,7 @@ defmodule Emily.MixProject do
       "-DMLX_BUILD_PYTHON_BINDINGS=OFF",
       "-DMLX_BUILD_SAFETENSORS=OFF",
       "-DMLX_BUILD_GGUF=OFF",
-      "-DUSE_SYSTEM_FMT=ON"
+      "-DMLX_BUILD_METAL_TESTS=OFF"
     ]
 
     # When xcode-select points at CommandLineTools (the default on fresh