Use ubuntu-22.04-arm-sdk docker image for test-backend-arm.yml (#19186)

Most vgf tests were failing with "Failed to create VkInstance". Running
with VK_LOADER_DEBUG=all revealed that the error was
[Vulkan Loader] ERROR | DRIVER: vkCreateInstance: Found no drivers!
The drivers are installed in the arm-sdk docker.

Working run here:
https://github.com/pytorch/executorch/actions/runs/25058662195

Author: Erik-Lundell

.github/workflows/_test_backend.yml

@@ -36,6 +36,11 @@ on:
         required: false
         type: string
         default: linux.4xlarge.memory
+      docker-image:
+        description: 'Docker image for Linux jobs'
+        required: false
+        type: string
+        default: ci-image:executorch-ubuntu-22.04-clang12
 
 jobs:
   test-backend-linux:
@@ -50,7 +55,7 @@ jobs:
     with:
       ref: ${{ inputs.ref }}
       runner: ${{ inputs.runner-linux }}
-      docker-image: ci-image:executorch-ubuntu-22.04-clang12
+      docker-image: ${{ inputs.docker-image }}
       submodules: recursive
       timeout: ${{ inputs.timeout }}
       upload-artifact: test-report-${{ matrix.flow }}-${{ matrix.suite }}

.github/workflows/test-backend-arm.yml

@@ -28,3 +28,4 @@ jobs:
       ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
       timeout: 120
       run-linux: true
+      docker-image: ci-image:executorch-ubuntu-22.04-arm-sdk