CMAKE_INSTALL_PREFIX in toolchain breaks compilation

I am setting CMAKE_INSTALL_PREFIX in a cmake toolchain for certain technical reason. In this case toolchain overrides command line parameters, so
https://github.com/ament/google_benchmark_vendor/blob/main/CMakeLists.txt#L66 has no effect and the installation logic below https://github.com/ament/google_benchmark_vendor/blob/main/CMakeLists.txt#L74 fails.

Is there a reason for installing external project to a temporary directory? It looks like an unnecessary step --CMAKE_INSTALL_PREFIX can be passed directly to the external project without extra install command. This would work just fine in my environment as well.