Arm backend: Update tag for ML SDK emulation layer. (#24791)

The tag was recently bumped in #24662, but the tag was removed.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils
@Sebastian-Larsson @robell @rascani

Signed-off-by: Erik Lundell <erik.lundell@arm.com>
Co-authored-by: Zingo Andersen <zingo.andersen@arm.com>

Author: Erik-Lundell

backends/arm/scripts/setup-mlsdk-from-source.sh

@@ -15,7 +15,7 @@ root_dir="${et_dir}/examples/arm/arm-scratch"
 setup_path_script=""
 mlsdk_manifest_dir="ml-sdk-for-vulkan-manifest"
 mlsdk_manifest_url="${MLSDK_MANIFEST_URL:-https://github.com/arm/ai-ml-sdk-manifest.git}"
-mlsdk_manifest_tag="${MLSDK_MANIFEST_TAG:-refs/tags/v2026.12.0.dev20261027}" # Keep this in sync with backends/arm/requirements-arm-vgf.txt and backends/arm/requirements-arm-vgf-runtime.txt
+mlsdk_manifest_tag="${MLSDK_MANIFEST_TAG:-refs/tags/v2026.09.0}" # Keep this in sync with backends/arm/requirements-arm-vgf.txt and backends/arm/requirements-arm-vgf-runtime.txt
 
 enable_model_converter=0
 enable_vgf_lib=0