Arm backend: Fix mypy linting in pre-push

[AdrianLundell]

• Adds ./src/ to MYPYPATH to find imports for editable install
• Make --revision point to the commit before the commit to be linted. Since lintrunner compares files diffing between --revision and HEAD, pointing it to the the latest commit (== HEAD) results in no files being linted.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• [PREEMPTIVE] Removal of ephemeral variants on scale-config.yml

❌ 1 New Failure

As of commit cc16926 with merge base dcd25eb ():

NEW FAILURE - The following job has failed:

• trunk / test-models-linux-aarch64 (linear, portable, linux.arm64.2xlarge) / linux-job (gh)
  RuntimeError: Command docker exec -t a7d5e313748f85384530706f4beef3e405b246e7de73131f5bbc0fc176f73a83 /exec failed with exit code 1

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[AdrianLundell]

Unrelated error trunk / test-models-linux-aarch64 (linear, portable, linux.arm64.2xlarge) / linux-job (gh): "error: incomplete-download"