ci: explicitly request msvc toolchain on windows
ci: explicitly request msvc toolchain on windows

The Travis CI Windows environment has changed in a way that causes CMake
to fail. Try using the MSVC toolchain instead.

thijsc/rust-rdkafka,fede1024/rust-rdkafka,thijsc/rust-rdkafka,fede1024/rust-rdkafka,fede1024/rust-rdkafka