[cmake] Explicitly disable download step

Fixes the following on cmake version 3.24.2:
'tools/clang/runtime/compiler-rt-stamps/compiler-rt-source_dirinfo.txt',
needed by 'tools/clang/runtime/compiler-rt-stamps/compiler-rt-download',
missing and no known rule to make it

Maybe related to https://cmake.org/cmake/help/latest/release/3.24.html#modules