SERVER-10906 StageBuilder set $language correctly for STAGE_TEXT

The type of expression on the right side of the assignment is
std::string.  Hence, 'language' was set to point inside a temporary
std::string constructed to hold the data from the result of c_str(),
instead of being set to point inside the the result of str() (which
has static lifetime).  Hence, 'language' became a dangling pointer.