`CMAKE_CUDA_FLAGS` is actually a string fragment and not a list

[tom91136]

The current CUDA models treats CMAKE_CUDA_FLAGS as a list when it's just a string, so any extra flags passed will have semicolons in place of spaces.
See https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS.html#variable:CMAKE_%3CLANG%3E_FLAGS:

This value is a command-line string fragment. Therefore, multiple options should be separated by spaces, and options with spaces should be quoted.

Pretty inconsistent CMake behaviour here but we just have to workaround it.
This breaks passing CMAKE_CUDA_FLAGS or CUDA_EXTRA_FLAGS explicitly.