[ET-VK][testing] Add ETVK_FORCE_NO_EXTENSIONS debug flag

[SS-JIA]

Stack from ghstack (oldest at bottom):

• -> [ET-VK][testing] Add ETVK_FORCE_NO_EXTENSIONS debug flag #17705
• [ET-VK][qconv] Add software fallback for dotPacked4x8AccSatEXT in q8ta_ shaders #17704
• [ET-VK][qconv] Read weight buffer as int in pack_q8_conv2d_weights shader #17703

Add a build config flag etvk.force_no_extensions that causes all
Vulkan extension support queries in Adapter.h to return false. This
makes it easy to test fallback code paths for devices that lack
optional extensions (e.g. 8-bit storage buffers, float16, dot product)
on a development host that would otherwise report support for them.

Enable with: --config etvk.force_no_extensions=1

Differential Revision: D94314257

[pytorch-bot]

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❌ 2 New Failures, 2 Unrelated Failures

As of commit c66108b with merge base 63f9724 ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/opt/android/sdk/platform-tools/adb' failed with exit code 224
• pull / unittest / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

• Test CUDA Builds / test-model-cuda-e2e (nvidia, parakeet-tdt, non-quantized) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, quantized-int4-tile-packed) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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