ci(mlx): install unsloth-zoo from git main on the macOS job

The macOS Apple Silicon job failed on its first run with

    NotImplementedError: Unsloth currently only works on NVIDIA, AMD
    and Intel GPUs.

surfaced from `unsloth_zoo.device_type.get_device_type()`. The cause
is the version pin: `pip install 'unsloth_zoo>=2026.5.1'` resolves
to the most recent PyPI wheel, which predates PR #620 and therefore
predates the `_is_mlx_only` gate in `unsloth_zoo/__init__.py` that
short-circuits the GPU device-type probe on Darwin+arm64+mlx.

Switch to `pip install --no-deps "unsloth_zoo @ git+https://github.com/unslothai/unsloth-zoo"`
so the macOS job sees the merged main branch and exercises the
actual MLX dispatch code. Studio's own `install.sh` does this for
exactly the same reason.

This is also the smoking gun the macOS runner exists to catch:
the spoofed Linux job cannot reproduce a stale PyPI/zoo pairing
because it never imports through device_type. The first real Mac
run found the gap on its first try.

.github/workflows/mlx-ci.yml

@@ -114,12 +114,19 @@ jobs:
       # without any spoofing -- exactly the conditions unsloth._IS_MLX
       # checks for. Once mlx is on the path the gate flips True and the
       # MLX import branch in unsloth/__init__.py runs.
-      - name: Install deps (real mlx + unsloth + unsloth-zoo)
+      #
+      # Install unsloth_zoo from git main rather than PyPI: the PR-A
+      # MLX support (#620) merged AFTER the most recent unsloth_zoo
+      # release, so the PyPI wheel still raises NotImplementedError on
+      # Apple Silicon at `device_type.get_device_type()`. Studio's own
+      # install.sh overlays unsloth-zoo from git main for the same
+      # reason; we mirror that here.
+      - name: Install deps (real mlx + unsloth + unsloth-zoo from git)
         run: |
           python -m pip install --upgrade pip
           pip install mlx mlx-lm
           pip install pytest pytest-asyncio
-          pip install 'unsloth_zoo>=2026.5.1'
+          pip install --no-deps "unsloth_zoo @ git+https://github.com/unslothai/unsloth-zoo"
           pip install -e . --no-deps
 
       - name: Verify _IS_MLX flips True on real Apple Silicon