The latest version of this document lives in the vcpkg repo.
Get the Program Files directory of the current platform's bitness:
either $ENV{ProgramW6432}
on 64-bit windows,
or $ENV{PROGRAMFILES}
on 32-bit windows.
vcpkg_get_program_files_platform_bitness(<variable>)
scripts/cmake/vcpkg_get_program_files_platform_bitness.cmake