[world rebuild][vcpkg scripts] Make vfap more robust

[dg0yt]

Do not simply fail on

set(DOXYGEN "")
if("docs" IN_LIST FEATURES)
  vcpkg_find_acquire_program(DOXYGEN)
endif()

with unexpected

CMake Error at scripts/cmake/vcpkg_extract_archive.cmake:19 (message):
  D:/downloads/tools/doxygen/8120346 was an extraction target, but it already
  exists.
Call Stack (most recent call first):
  scripts/cmake/vcpkg_find_acquire_program.cmake:212 (vcpkg_extract_archive)
  ports/dpdk/portfile.cmake:67 (vcpkg_find_acquire_program)
  scripts/ports.cmake:206 (include)

(#48226)

[Neumann-A]

Needs to be a existing path so that if(EXISTS "${PATH}") can be used in an implementation. Using an arbitrary string which is not a valid and existing path should be forbidden.

[dg0yt]

I see the direction.
This is is an input value here. Both vfap and find_program short-cut for any trueish value. So the proposed requirement isn't used at the moment. I'm not convinced that it is necessary to establish it in the test now when I can't be sure that some ports don't already expect the current behavior, e.g. due to managing PATH and program variable.