fix dangling pointer warning
fix dangling pointer warning

fixes:
external/jsonnet/cmd/jsonnet.cpp:298:31: warning: object backing the pointer will be destroyed at the end of the full-expression [-Wdangling-gsl]
            const char *arg = next_arg(i, args).c_str();
                              ^~~~~~~~~~~~~~~~~
sparkprime/jsonnet,sparkprime/jsonnet,google/jsonnet,sparkprime/jsonnet,google/jsonnet,sparkprime/jsonnet,google/jsonnet,google/jsonnet,sparkprime/jsonnet,sparkprime/jsonnet,google/jsonnet,google/jsonnet