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 b725a19072 (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

Modules/FindMPI.cmake

@@ -1157,7 +1157,7 @@ macro(_MPI_create_imported_target LANG)
   string(REPLACE "-pthread" "$<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:SHELL:-Xcompiler >-pthread"
     _MPI_${LANG}_COMPILE_OPTIONS "${MPI_${LANG}_COMPILE_OPTIONS}")
   string(REPLACE "-fexceptions" "$<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:SHELL:-Xcompiler >-fexceptions"
-    _MPI_${LANG}_COMPILE_OPTIONS "${MPI_${LANG}_COMPILE_OPTIONS}")
+    _MPI_${LANG}_COMPILE_OPTIONS "${_MPI_${LANG}_COMPILE_OPTIONS}")
   set_property(TARGET MPI::MPI_${LANG} PROPERTY INTERFACE_COMPILE_OPTIONS "${_MPI_${LANG}_COMPILE_OPTIONS}")
   unset(_MPI_${LANG}_COMPILE_OPTIONS)