Bump to microsoft/vcpkg@c8696863d3 (#133)

Changes: microsoft/vcpkg@94ce0da...c869686

A recent build attempt failed downloading various mingw dependencies:

    Error: Failed to download from mirror set:
    https://repo.msys2.org/mingw/i686/mingw-w64-i686-libwinpthread-git-9.0.0.6373.5be8fcd83-1-any.pkg.tar.zst: failed: status code 404

Bumping to the latest vcpkg release tag appears to fix this.