CheckLanguage: Fix forwarding of CMAKE_CUDA_HOST_COMPILER

Fix the condition added by commit fada8cbf (CheckLanguage: Report
CMAKE_CUDA_HOST_COMPILER if needed for compilation, 2019-05-31,
v3.15.0-rc1~12^2) to activate CUDA-specific logic.  The old condition
had worked in our test suite only by accident because the loop variable
used in the test happened to be the name and value that the old
condition used!  Update the test to use a different name.

Fixes: #19013