Fix runner_utils path in Arm backend

[AdrianLundell]

The import accidentally didn't include the full path which caused issues finding the module in some cases.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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❌ 1 New Failure, 13 Pending

As of commit 651be1a with merge base 6e0cb06 ():

NEW FAILURE - The following job has failed:

• trunk / test-models-macos (cmake, mobilebert, xnnpack-quantization-delegation, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Failed to import transformers.models.mobilebert.modeling_mobilebert because of the following error (look up to see its traceback):

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