CI: cache SwiftShader prebuilt to skip per-run from-source build

[JulianCloudNTH]

Summary: Try a pinned-rev SwiftShader prebuilt from gha-artifacts before building from source (mirrors the torch-wheel download-or-build in .ci/scripts/utils.sh); on a cache miss, fall back to the existing from-source build. The key embeds SWIFTSHADER_REV, so a rev bump auto-invalidates (re-seed S3 once per bump). Follow-up to the Dawn CI diff, addressing review feedback to cache the build.

Differential Revision: D108185797

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20203

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❌ You can merge normally! (1 Unrelated Failure), 2 Unclassified Failures

As of commit d6e9a01 with merge base f0dff03 ():

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Build Windows Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Process completed with exit code 1.
• Build Windows Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x64

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

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

• pull / android / build-android (gh) (trunk failure)
  Process completed with exit code 1.

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[meta-codesync]

@JulianCloudNTH has exported this pull request. If you are a Meta employee, you can view the originating Diff in D108185797.

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