Remove unnecessary std::move when constructing RapidYamlError

[johnbartholomew]

From a compile warning:

jsonnet/core/vm.cpp:3484:30: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
 3484 |         throw RapidYamlError(std::move(msg.str()));
      |                              ^
jsonnet/core/vm.cpp:3484:30: note: remove std::move call here
 3484 |         throw RapidYamlError(std::move(msg.str()));