Cadence: Add nightly schedule to build and test workflow

[aliafzal]

Summary

• Add a cron schedule (2 AM UTC daily) to the Cadence Build & Test workflow so it runs as a nightly job
• Keeps existing PR and push-to-main triggers

Test plan

• Verified workflow YAML syntax is valid
• Schedule follows the same pattern as other nightly backends (arm, coreml, xnnpack, etc.)

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18750

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures, 2 Cancelled Jobs, 1 Pending, 2 Unrelated Failures

As of commit bf0374e with merge base 71db7c7 ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t bb04b1ac61d02090b631945a9ec3856b33ae703e00993a4e2b85f0ea0fb222a5 /exec failed with exit code 139
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• Build Presets / windows (windows) / build (gh)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / unittest-editable / windows / windows-job (gh)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-llama-runner-linux (fp32, xnnpack+custom+qe, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / unittest / windows / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

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[github-actions]

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[meta-codesync]

@aliafzal has imported this pull request. If you are a Meta employee, you can view this in D99908360.