cmake: ensure that the `vcpkg` packages are found on Windows

cmake: ensure that the `vcpkg` packages are found on Windows

On Windows, we use the `vcpkg` project to manage the dependencies, via
`compat/vcbuild/`. Let's make sure that these dependencies are found by
default.

This is needed because we are about to recommend loading the Git
worktree as a folder into Visual Studio, relying on the automatic CMake
support (which would make it relatively cumbersome to adjust the search
path used by CMake manually).

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>