cmake : fix ui build with read-only source

[o7si]

When building out-of-tree against a read-only source, UI provisioning runs npm in the source tree, so it fails creating node_modules there. The fix stages the UI sources into a writable copy under the build dir and runs npm there, leaving the source tree untouched.

Related issue:

• Compile bug: out-of-tree build fails to provision UI assets: ENOENT: no such file or directory, mkdir '/src/tools/ui/node_modules' #24745

[o7si]

Tested on macOS (arm64) — out-of-tree build from a read-only source:

git clone --depth 1 https://github.com/ggml-org/llama.cpp.git /tmp/llama-ro
git -C /tmp/llama-ro fetch --depth 1 origin pull/24752/head
git -C /tmp/llama-ro checkout FETCH_HEAD
rm -rf /tmp/llama-ro/.git
chmod -R a-w /tmp/llama-ro
cmake -S /tmp/llama-ro -B /tmp/llama-build -DLLAMA_BUILD_UI=ON -DLLAMA_USE_PREBUILT_UI=OFF
cmake --build /tmp/llama-build --target llama-ui

Full build log: https://gist.github.com/o7si/c466d6f6dfa3320feb58fa662af52b90