Use proper vcpkg vars for detecting build environment OS

This should fix these errors:

Make Warning at scripts/ports.cmake:167 (message):
  Unexpected UNKNOWN_READ_ACCESS on variable APPLE in script mode.

  This variable name insufficiently expresses whether it refers to the target
  system or to the host system.  Use a prefixed variable instead.

  - Variables providing information about the host:

    CMAKE_HOST_<SYSTEM>
    VCPKG_HOST_IS_<SYSTEM>

  - Variables providing information about the target:

    VCPKG_TARGET_IS_<SYSTEM>
    VCPKG_DETECTED_<VARIABLE> (using vcpkg_cmake_get_vars)

Call Stack (most recent call first):
  C:/code/ThirdParty-Packages-vcpkg/custom-ports/whispercpp-basic/portfile.cmake:2147483647 (z_vcpkg_warn_ambiguous_system_variables)

custom-ports/whispercpp-basic/portfile.cmake

@@ -9,7 +9,7 @@ vcpkg_from_github(
         0001-UpdateTargetName.patch
 )
 
-if(APPLE)
+if(VCPKG_HOST_IS_OSX)
     vcpkg_cmake_configure(
         SOURCE_PATH ${SOURCE_PATH}
         OPTIONS

custom-ports/whispercpp/portfile.cmake

@@ -7,7 +7,7 @@ vcpkg_from_github(
     HEAD_REF master
 )
 
-if(APPLE)
+if(VCPKG_HOST_IS_OSX)
     vcpkg_cmake_configure(
         SOURCE_PATH ${SOURCE_PATH}
         OPTIONS