[build Folder Migration] Alert for core scripts

[jathu]

Summary

Given the move in #9434, lets keep the scripts around for a week before deleting them.

Test plan

Ran them.

$ build/build_android_library.sh

[error] build/build_android_library.sh has moved to: scripts/build_android_library.sh

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/9545

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 2 New Failures, 5 Pending, 1 Unrelated Failure

As of commit 34f9da7 with merge base 90f0843 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for build/print_exported_headers.py:
• pull / unittest-editable / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_check_quant_params.py::TestCheckQuantParams::test_inject_invalid_zp

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

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
  backends/xnnpack/test/passes/test_convert_to_linear.py::TestConvertToLinear::test_fp32_convert_to_linear

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[mergennachin]

Since we're cutting our branch 0.6 soon, let's just move forward and not have this intermediate stage, and announce in the release notes