The documentation for CMAKE_MSVC_RUNTIME_LIBRARY states [1]:

> This variable has effect only when policy CMP0091 is set to NEW prior to
> the first project() or enable_language() command that enables a language
> using a compiler targeting the MSVC ABI.

so the current usage of CMAKE_MSVC_RUNTIME_LIBRARY for vcpkg does not work
at all.

Let's fix that by setting policy 91 to new if present.

[1]: https://cmake.org/cmake/help/latest/variable/CMAKE_MSVC_RUNTIME_LIBRARY.html