Compile bug: Post-refactor issues wth Server/UI builds

[allozaur]

Parent issue for few edge cases that have appeared after merging #22937

[mrunge]

I ran into

CMake Error at /builddir/build/BUILD/llama.cpp-b9265/scripts/xxd.cmake:10 (file):
  file failed to open for reading (No such file or directory):

    /builddir/build/BUILD/llama.cpp-b9265/build/tools/ui/dist/loading.html

I passed

    -DLLAMA_BUILD_UI=ON \
    -DLLAMA_USE_PREBUILD_WEBUI=OFF

to cmake. Is that something you could look into as well?

[allozaur]

@mrunge please check with changes from #23352