ci(backends): build cpu-whisperx and cpu-faster-whisper for linux/arm64 (#9573)

Extend the existing CPU build matrix entries to produce a multi-arch
manifest (linux/amd64,linux/arm64) at the same image tags. arm64
Linux hosts without an NVIDIA GPU report the "default" capability,
which already maps to cpu-whisperx / cpu-faster-whisper in
backend/index.yaml -- so the manifest list lets Docker pull the right
variant without any gallery changes.

Both stacks install cleanly under aarch64: torch (2.4.1/2.8.0),
faster-whisper, ctranslate2, whisperx, opencv-python and the
remaining deps all ship manylinux2014_aarch64 wheels, so no source
builds run under QEMU emulation.

Follows the same pattern already used by cpu-llama-cpp-quantization.

Assisted-by: Claude:claude-opus-4-7 [Claude Code]

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

.github/workflows/backend.yml

@@ -141,7 +141,7 @@ jobs:
           - build-type: ''
             cuda-major-version: ""
             cuda-minor-version: ""
-            platforms: 'linux/amd64'
+            platforms: 'linux/amd64,linux/arm64'
             tag-latest: 'auto'
             tag-suffix: '-cpu-whisperx'
             runs-on: 'ubuntu-latest'
@@ -154,7 +154,7 @@ jobs:
           - build-type: ''
             cuda-major-version: ""
             cuda-minor-version: ""
-            platforms: 'linux/amd64'
+            platforms: 'linux/amd64,linux/arm64'
             tag-latest: 'auto'
             tag-suffix: '-cpu-faster-whisper'
             runs-on: 'ubuntu-latest'