Added gcc 9.1 test

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@markdroth This PR adding gcc 9.1 to portability test is ready so you can start working on having a workaround of std::variant (#30140)

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grpc/core/master/linux/grpc_portability: running

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Portability Test now can reproduce this error by run_tests_c++_linux_dbg_native_x64_gcc9_1_buildonly

/usr/local/include/c++/9.1.0/variant:609:6: error: no matching function for call to std::variant<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::vector<grpc_core::XdsRouteConfigResource::Route::RouteAction::ClusterWeight, std::allocator<grpc_core::XdsRouteConfigResource::Route::RouteAction::ClusterWeight> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::variant(const std::__cxx11::basic_string<char>&)

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