[cadence] Run hifi op-test nightly only, not on every PR

[aliafzal]

Gate the hifi-op-test job to run on the nightly schedule (plus post-merge push to main/release and manual workflow_dispatch) and NOT on pull_request / pull_request_target, so the licensed Xtensa toolchain + xt-run cost is not paid on every PR.

The existing nightly cron (0 8 * * *) and the post-merge push run catch regressions. cpu-build/cpu-test, hifi-build, and vision-build are unchanged (still run per-PR).

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20749

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❌ 4 New Failures, 4 Pending, 1 Unrelated Failure

As of commit b30ebc2 with merge base b32c2c3 ():

NEW FAILURES - The following jobs have failed:

• pull / test-binary-size-linux-gcc / linux-job (gh)
  RuntimeError: Command docker exec -t 12439a1e88c026cf388633efbc106ab4e3a0ec34dda3ecced600cf02082b3064 /exec failed with exit code 1
• pull / test-qnn-models-linux (mv2) / linux-job (gh)
  RuntimeError: Command docker exec -t 5a0566ad2dd7717ac0d9704e821146ae25aeba5c3cb41c5d640aa423b601b408 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 5fdf3bb125ae713c0e93b0a3762e1e10242bc70bbc99da50fada8403b1cc09f4 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 15358865faa5a0c6e6d187b91c4cd96e85db219e816d48ba665c5a65e1b42f3f /exec 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)

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