Cap PyTorch build parallelism for all GCC docker images

[rascani]

Summary

The gcc14 docker build was intermittently timing out on linux.4xlarge runners because it built PyTorch from source with unlimited parallelism, unlike gcc11 which capped MAX_JOBS=6. Generalize the guard to all GCC variants so gcc14, gcc15, and future additions get the same protection.

Images that set SKIP_PYTORCH (gcc9-nopytorch, cuda-windows) are unaffected because the existing SKIP_PYTORCH guard excludes them.

Fixes #19881

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20123

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❌ 2 New Failures, 6 Cancelled Jobs

As of commit 85a114c with merge base ac3003e ():

NEW FAILURES - The following jobs have failed:

• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, models) / linux-job (gh)
  RuntimeError: Command docker exec -t 3e160a93cb00746cb54bfbd5ffc74709c76b606a0fc744bd7a43f6c71b04d465 /exec failed with exit code 1
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t bf08c24f43ca85b46a8eec2d0c9d66b18b5a59e6c32533f1040a49c3848a5ee4 /exec failed with exit code 1

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

• pull / test-lora-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-qnn-passes-linux / linux-job (gh)
• pull / test-qnn-python-imports-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / unittest / linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / unittest-editable / linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / unittest-nxp-neutron / linux-job (gh)
  ##[error]The operation was canceled.

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