fix(common): compile with GCC (MinGW-w64) under Windows (#9140)
fix(common): compile with GCC (MinGW-w64) under Windows (#9140)

The use `_dupenv_s()` is a MSVC requirements, it is not needed for GCC
under Windows. Note that we do not plan to test systematically with this
compiler.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp