Fix race condition in CMake code. (#1555)

The race is cause by the fact that two targets exist to copy the Python
files. The first one because of `cpp_cc_build_time_copy` without
`NO_TARGET`. The second because of the:

    add_custom_target(nmodl_copy_python_files ALL DEPENDS ...)

The solution is to not create the per file target by passing
`NO_TARGET` to `cpp_cc_build_time_copy`.

src/pybind/CMakeLists.txt

@@ -99,7 +99,7 @@ file(
 
 foreach(file IN LISTS NMODL_PYTHON_FILES)
   cpp_cc_build_time_copy(INPUT ${NMODL_PROJECT_SOURCE_DIR}/python/nmodl/${file} OUTPUT
-                         ${CMAKE_BINARY_DIR}/lib/nmodl/${file})
+                         ${CMAKE_BINARY_DIR}/lib/nmodl/${file} NO_TARGET)
   list(APPEND nmodl_python_binary_dir_files "${CMAKE_BINARY_DIR}/lib/nmodl/${file}")
 endforeach()
 add_custom_target(nmodl_copy_python_files ALL DEPENDS ${nmodl_python_binary_dir_files})