[llvm] [docker][Windows] install libxml2 using vcpkg (PR #209422)

[Charles Zablit via llvm-commits]

charles-zablit wrote:

> Can we not install this with `choco`?

Unfortunately, it's not available on choco: https://community.chocolatey.org/packages?q=libxml2

https://github.com/llvm/llvm-project/pull/209422