internal_os_Zygote: Avoid string copy

Since PrepareDir() takes a const std::string&, we just want to
directly pass our std::string, instead of passing the result
of c_str(), which would require constructing a new std::string.

Test: TreeHugger
Change-Id: Ifecd7ce61f5cd744c1562600cfe9f69353e6d80e