FindMPI: Fix regression in pthread guard

Fix a regression with MPI and CUDA<10.2 that did let `-pthread` flags
slip to nvcc again.  In commit b725a190 (FindMPI: Deny -fexceptions
from NVCC, 2020-07-02, v3.18.0-rc4~12^2) we accidentally forgot to use
the variable containing the replacement result.

Fixes: #21108