Fix Linux source archive dependency cleanup

[drebbe-intrepid]

Linux builds from source archives fail because git clean runs outside the nested dependency checkout. Clean and recreate only LIBICSNEO_BUILD, preserving dependency sources, installation files, and the protobuf bootstrap cache.

Fixes #248.

Validation:

• Reproduced the original Git exit 128 using a temporary archive-style tree and a real nested Git checkout.
• Three regression tests pass on Windows CPython 3.12 and WSL Ubuntu: fresh archive-style build, scoped rebuild cleanup/preservation, and cleanup-error propagation. CMake calls are stubbed; no full source compilation or hardware tests were run.
• git diff --check passes.
• Broader unittest discovery cannot import the unbuilt ics extension in this fresh worktree (six import errors).

Independent critic /root/critic explicitly agreed with the final two-file diff: "I agree with the final diff for issue #248. No required corrections." Builder also explicitly agrees. Reviewed content is committed as 2d0eb35.