ci(darwin): build the ds4 backend for darwin/arm64 (metal)

The gallery has metal-ds4 / metal-ds4-development entries, and the build
recipe exists (make backends/ds4-darwin, special-cased in
backend_build_darwin.yml), but ds4 was never listed in the darwin matrix,
so no metal-darwin-arm64-ds4 image was ever published and the entries
dangled. Add ds4 to the darwin matrix, mirroring the llama-cpp form (the
reusable workflow builds it via 'make backends/ds4-darwin').

sherpa-onnx and speaker-recognition are already present in the darwin
matrix on master and are not changed here.

Assisted-by: claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

Author: mudler

.github/backend-matrix.yml

@@ -4475,3 +4475,6 @@ includeDarwin:
   - backend: "speaker-recognition"
     tag-suffix: "-metal-darwin-arm64-speaker-recognition"
     build-type: "mps"
+  - backend: "ds4"
+    tag-suffix: "-metal-darwin-arm64-ds4"
+    lang: "go"