Fix TOSA serialization install in Linux unittest

[JacobSzwejbka]

Summary:

Fix the Linux unittest TOSA serializer install by moving the fallback clone to tosa-tools v2026.05.0 and installing from the tosa-tools repo root. Upstream moved packaging to the root pyproject in this release; installing from serialization/ is now intentionally unsupported.

This avoids patching pyproject.toml in CI and uses the current upstream packaging layout instead.

Authored with Claude.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20750

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❌ 2 New Failures, 2 Unrelated Failures

As of commit 704d32f with merge base 6edd49d ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 74600852654e45d7cebe4d762dc0aa1d4e7f3f93d33fbd35bbc45861fc9b2fc9 /exec failed with exit code 139
• trunk / test-arm-backend-ethos-u (test_pytest_ops_ethos_u55) / linux-job (gh)
  RuntimeError: Command docker exec -t 6ae16d8155182ecc14a81f1c9c1ecd4b5f4ad5804a47634c9616b2b70adb3604 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux (resnet50, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-nxp-neutron / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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