[Backend Tester] Add nightly CI job for XNNPACK

[GregoryComer]

Set up a nightly CI job to run the backend model and operator tests for xnnpack (unquantized and static int8 per channel). This is the first step in setting up continuous coverage using the new suite. I've chosen nightly as the initial run cadence as some of the larger test suites are very long. In the medium-term, I'd like to run a subset of these tests on pull or trunk, but we can start with this.

[GregoryComer]

Stack from ghstack (oldest at bottom):

• [Backend Tester] Mark adaptive avgpool2d as an unsupported portable op #13398
• -> [Backend Tester] Add nightly CI job for XNNPACK #13390
• [Backend Tester] Add test flow CLI arg #13360

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13390

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❌ 5 New Failures, 2 Cancelled Jobs

As of commit 600cf8a with merge base 30a6f5e ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• nightly / backend-test-linux (xnnpack_static_int8_per_channel, models) / linux-job (gh)
  RuntimeError: Command docker exec -t 3ea06ff905a01daf263bcb9129cefc7d085e3f53c3fc0e1b4c53bc5e4ed44b8c /exec failed with exit code 1
• nightly / backend-test-linux (xnnpack_static_int8_per_channel, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t 670e2fcc11d15d92e33dde8ba6eb70bb6844317e32e2a174400332cfde22c970 /exec failed with exit code 1
• nightly / backend-test-linux (xnnpack, models) / linux-job (gh)
  RuntimeError: Command docker exec -t 6e38cf812942feb9ffe42f20d0938de8031d6452d45e478986d808ed693c7d54 /exec failed with exit code 1
• nightly / backend-test-linux (xnnpack, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t eb8cd87572614cdb3b6b58e76d655dc53b09f8a02f6c1fcc202786cceff7139b /exec failed with exit code 1

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

• pull (gh)
• trunk (gh)

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