Arm backend: skip add vfg test if not vkml executor runner found

[mansnils]

Fix for CI environments that are not setup yet.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13864

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❗ 1 Active SEVs

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 104681d with merge base 899d7e5 ():

NEW FAILURE - The following job has failed:

• trunk / test-llama-runner-mac (fp32, coreml) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-nxp-neutron / linux-job (gh) (trunk failure)
  test_split_group_convolution__applied_by_default

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